Modeling 101 A Guide to Building a Travel

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Modeling 101
A Guide to
Building a
Travel
Demand Model
in TransCAD
By Rhett Fussell
&
Leta Huntsinger
Model Research & Development Unit
NC Department of Transportation
Statewide Planning Branch
What is Modeling 101??
It is an attempt by the Model Development and Research Unit to create a step by step
reference manual/class for users in the Statewide Planning Branch. The intent of the
manual is to teach users the following things:
1) The process of how to build a Travel Demand Model in Transcad
2) Easy checks and tips on how to get the best results from your model while you are
building
3) A guide to look back at when you need to refresh yourself on steps in the process
4) Brief discussions on the theory behind modeling
Tutorial files are included on the network drive as well as on CD for your use. The
directory structure is setup to make sense in the modeling process. There is a folder for
each step of the process and a few others that help keep files organized in the proper
location. It is strongly encouraged that you follow this same structure to make it easier
for you or others who may assist you along the way.
With all of these things combined into one manual it will
allow you to become the State’s best modeler in no time
at all!!!!!
(Disclaimer: Results may vary and Not the Cindy crawford type model of Course!!)
Lets get started . . . . . . . .
.....
Modeling 101
Chapter 1
Before You Can Begin Building the Model
In order to build the actual travel demand model there are some things that you
must understand and steps that must be completed before the actual modeling
work can be done.
Just like you when you were a baby you couldn’t walk before learning to crawl!
Below is a list of things that you need to have
completed before beginning to build your model:
1) Be familiar with the basic commands in TransCAD –you
should have completed the Intro to TransCad GIS notes before beginning this
modeling portion of TransCad
(ie….adding links, editing, layers, select by condition, etc!)
2) Clean Linework – this is a file that ONLY includes the roads you will be
MODELING!(refer to the GIS notes section)
*Links connected(using connectivity tool)
*Node ID’s changed to match your zones (See addendum in back of notebook)
*Segments merged, dualized & simplified (See addendum in back of notebook)
*at a minimum these attributes coded correctly in your data table:
Note: These fields are necessary to use the TransCad tools that SWP is or will be developing to
make the process easier. You MUST have a column for them in your data table even if you
don’t have information in the column!!! It is important to note that you must spell the field
names exactly the way they are shown in the dialog box above! The one capacity is
automatically calculated for you so you just need the two way column in your layer. It will
recognize one way links and calculate the capacity correctly.
3) Archive of your linework
Chapter 1 – Introduction
1
Modeling 101
Other Info You Might Want to Have
Zone Map
Background Roads Layer
Environmental Data
Aerial photos
Etc……….
Travel Time Calculation
It is important that you have already calculated travel time as part of your linework
cleaning as mentioned in bullet two on this page. There is a tool that was developed to
calculate the travel time on your network for you.
First you must install the program on your local drive (do it only once)
1. Go to S:\TransCAD\GISDKtools\swpTools\r1 and double click on file
swpToolsSetup.bat
Or on the CD go to \GISDKTools\swptools\r1
This will install the program on your local drive at
C:\Program Files\TransCAD\myGISDK
Note: For future releases the r1 may change to r2, r3 etc…..
Then you must add the program in TransCAD (do it only once)
1. In TransCAD, go to Tools-Add Ins menu
2. In Add-ins dialog box, click button Setup…
3. Click Add to display the Setup Add-ins dialog box
4. Fill out all the boxes to look like this(it is CASE SENSITIVE so make sure you type
the values in correctly.)
SWP Tools(this can be whatever you want to name it)
swptools
C:\Program Files\TransCAD\myGISDK\swptools.dbd
Chapter 1 – Introduction
2
Modeling 101
Now you can run the program in order to calculate your travel time (there are other
tools that can be run using this toolbox as well)
1. In TransCAD, go to Tools-Add Ins menu
2. Double click on SWP Tools in the Add-ins list
You should now see a new menu called SWP-Tools appear on the menu bar :
and a new toolbox appear on screen similar to this:
Click on the Calc TTIME button and the travel time for your network will be added to
your geographic file permanently!
If you change a speed limit then the travel time is automatically re-calculated.
If you add new links you must use the button again!
So now you’re asking yourself, what is my travel time
unit…….If you guessed MINUTES you are right!
Give yourself a high five and move on to Chapter 2!!
Chapter 1 – Introduction
3
Modeling 101
Chapter 2 - Building Your Network
Once you have clean linework you are now ready to create your network and begin the
modeling process.
Before you attempt this step it is IMPORTANT for you to UNDERSTAND the
difference between “linework” and “network”.
1)Open the map Pilot Mtn Roads.map
(located in the Base Year folder of the Pilot Mtn data)
Linework – This is all the roads you might have in your area that have attribute data
associated with them. This attribute data in the linework may be just for reference or
mapping or data you want to keep for future use. For example you might have columns
in your data table like road names, # of lanes, functional class & type of future
improvement. This layer might look like this:
And your data table might look like this for the linework:
Chapter 2 – Building Your Network
4
Modeling 101
Network:
The network is based on the linework but includes only the LINKS YOU WANT TO
MODEL!!! The network is the links in which you want to use the travel demand model
to predict volumes and to solve transportation problems. It is a scaled down set of roads
that is based on a geographic linework layer. This layer would look like this:
Network file being used is
listed here!!
Note: This is a geographic file displayed here but the “network” is actually just a reference file that is
used in the model process. The name of the network being used in modeling is noted at the bottom of the
TransCAD window.
The main concept to take away is that the network is used to analyze the roadways and
the linework(geographic file) is used to display the results!!!
Well, now that you know the difference between linework and network let’s make our
network!!
Close the map you currently have open
Step 1 – Open the geographic file that you want to base your model network on. This
file can be a layer with only the network roads on it(which is recommended) or it can be
the larger linework file that you can use the Select by Condition command on, to get the
network roads. We will be doing the steps based on having a geographic line file that has
just the network roads on it, similar to what was shown in the previous graphic above!
Open the file pilot mtn base year linework.dbd
Step 2- Go to the Networks\Paths Menu (If you do not see this menu at the top of the screen
then go to Procedures and click on Networks/Paths. The new menu should now appear) and Select
CREATE
Chapter 2 – Building Your Network
5
Modeling 101
You should now see this pop-up screen:
What part of the geographic
file to use to create the
NETWORK
Description of the network you
are creating
Select the fields you want
included in the network.
These are the fields that
will be used in your
analyzing and loading of
the network!
Make sure this is checked
*See assignment type
section below that
discusses what fields
are needed for
modeling
Gets rid of duplicate links still in file
after cleaning it!
Click OK and Name the file MY Network
You have created your network!
Step 3 - Check to make sure you actually created the network and that it is referenced
correctly by looking at your window and seeing what file is listed. As shown below:
Network file name
should be listed here!
NOTE 1: The active network is kept in memory and used for all modeling analysis until
you change it to another network. It is important that you check which network is being
referenced before you perform any analysis!!! To close the network file Right Click over
the file name and select Close All!
Chapter 2 – Building Your Network
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Modeling 101
NOTE 2:
a) To change networks go to the File-Open menu to display the dialog box
b) Select Network as file type and then choose the network file you want to
use and click OK
c) The new network file should be displayed at the bottom of the screen.
Assignment Type Section – This is not intended to discuss in detail the type of
assignment technique you should use but gives a starting point for the types of info that
should/will be needed to perform various tasks associated with assignment of the trips
onto the network.
At a minimum it is recommended that travel time and one way capacity should be
included as data fields in your network.
Travel Time is a field that has to be calculated by you, the user. If ANY changes
are made to your linework you must recalculate travel time and
create a new network file in order for the changes to be used in the
analysis. If you do not re-specify your network then your model analysis will
be incorrect. The steps should be performed as described previously! In the
future this will be an automatic function that is performed by clicking a button,
but for now you will have to use the fill command on your travel time column
as mentioned in Chapter 1.
The capacities that are used in modeling for TransCAD MUST be ONE WAY! TransCAD
recognizes, in its internal memory, each link as a one way link even though it does not
show up on screen that way. Therefore if you do not use the right capacity then the
traffic assignment could be incorrect on your model.
For most of our small urban models we use All-Or-Nothing assignment but we will be
teaching you other techniques in the future. We encourage you to adopt as standard
practice a technique that better represents reality, unlike the all-or-nothing technique,
which is best for representing pure demand. For this reason it is good to be aware of the
other techniques and the fields that are necessary. Here is the table for the data fields
needed depending on the type of model loading technique you will be using. The loading
technique will be discussed in more detail later.
All-Or
Nothing
Stochastic
Capacity
Restraint
User Equilibrium &
Stochastic User Equilibrium
Travel Time
(Calculated using
speed field)
Capacity
(One Way)
Facility Type
Chapter 2 – Building Your Network
7
Modeling 101
Now that we have created our network we need to review the settings on the network and
do some logical error checking before we move on to building our minimum paths and
trip generation.
So let’s start with the settings menu which is located under
Networks/Paths Æ Settings
You should get this dialog box:
The active network file name
is listed here! Or you can
select a different network by
clicking on the dots
Name of linework
that the network is
based on!
Fields that are in the
network and can be
used for analysis
…you selected these
earlier
General info about your network.
Check to see if it’s correct!!!!
This box is used to tell TransCAD not
to let the centroids be used as a path
to cross the network. You should
always check this box.
After you have checked through this screen, click on the Other Settings tab.
Chapter 2 – Building Your Network
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Modeling 101
This is where we tell it
how to determine what
nodes are centroids! See
the Selecting Centroids
section below for the
steps!
Selecting Centroids
A centroid is a point in a Transportation Analysis Zone that represents the data in the
zone. In order for TransCAD to know what the centroids in our network are we need to
create a selection for the centroids only. As noted in the text box on the figure on the
previous page we do not want our centroid links to be used as a “shortest path” when we
build our travel time/skim matrix, so we must determine which nodes not to use. If we
tell TransCAD that there are centroids and there is a selection set called that, it will
recognize this fact when assigning trips. You also need to specify centroids for building
the zone-to-zone travel time matrix. This is called multiple shortest paths and will be
covered in future sessions.
Step 1 - It is important that you have changed your node ID’s to represent
the actual zone numbers for your area. (this is optional but encouraged)
This was covered previously and the notes are located on the Share drive in the
TransCADÆUser’s Group Notes folder.
They are called:
“TransCAD Notes for nodes & centroids.pdf”
Step 2 - We now need to show our nodes. Go under Map Æ Layers. The Layers
dialog box will come up. Highlight Nodes and click on "Show Layer". Click Close. The
network should redraw with the nodes highlighted.
Chapter 2 – Building Your Network
9
Modeling 101
Make sure that the Endpoints is the Working Layer at the top of the screen
(as shown in the figure).
Step 3 - We now need to make a Centroid set.
Choose SelectionÆ Select by Condition to display that dialog box.
We now need to build a formula for TransCAD to tell it which nodes are centriods. The
first 60 nodes of this model are centroids or external stations, so any node number less
than or equal to that number is a centroid. Just for ease of remembering I will use 60 as
my reference number! You will have to know what your highest centroid number is
(including your external stations) and that is the number you should use in the
equation for the selection condition.
You can either type the formula in the "Enter by Condition" box or use the Condition
Builder to select your condition. Under "Field List" use the scroll down menu to find and
click on ID. Under "Operator List" use the scroll down menu to find and click on <=.
Now go into the "Enter By Condition" box and type in 60, so the formula should be
ID <= 60.
*Or you may create a column in your data table that designates that link as a centroid.
Place a 1 in that column where centroid links are located and then create a selection set
based on that column.
Chapter 2 – Building Your Network
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Modeling 101
We need to give this selection set a name. Under "Set Name" rename this box Centroids.
Your screen should be similar to this:
Click OK. Your network should redraw, and the centroids will change color (in my case
green). Our centroids are now identified.
Now we can go back to our Other Settings dialog box and tell TransCAD which nodes
are centroids.
Step 4- Make sure you are still in the node working layer.
A) Choose Networks/Paths Æ Settings to display the Network Settings dialog
box.
B) Click the box next to Centroids in the "Options" box.
C) Go to the Other Settings Tab
In this dialog box, there is a "Centroids are" box. Inside that, choose In Selection Set,
and Centroids in the drop down menu. After you do this, you should see "In Network (46
Nodes)" pop up- or whatever your total number of centroids came out to be.
Click OK.
NOTE to SELF: You should not have created any “Dummy Zones” when you built
your network. This will confuse TransCAD when it is building the matrices. You can
add zones later if you need to so don’t worry about it for now!!!
Final Checks on Your Network:
After doing the necessary steps to create your network it is important to do some error
checking to ensure that this step of the travel demand model is as accurate as possible.
By doing these checks now you will be able to better understand what you just completed
and you will find it easier to remember the details of the network coding. The TransCAD
GIS class sessions should give you the necessary background to create themed plots,
selection sets, etc.
Chapter 2 – Building Your Network
11
Modeling 101
Here’s a list of a few of things you should go through BEFORE moving on to minimum
paths and trip generation!!!!
Highway Networks
ƒ Centroid connectors should represent as closely as possible the local street system.
Use GIS tools and local knowledge to locate centroid connectors.
ƒ Size and density of the zones should correspond to the level of detail of the coded
highway network. Major physical barriers should not split zones. Use GIS and local
knowledge to check.
ƒ Network review should include visual inspection in addition to range checking for
capacities, speeds and distances.
ƒ Use minimum path techniques to check for coding errors in the link attribute
impedance factors.
ƒ Network attributes should be plotted and checked (distances, speed limit, facility
classification, area type and number of lanes).
Tutorial Assignment
A) Error-check the network for shortest paths. Use the shortest path tool to perform
some logical error checks.
To use the shortest path tool go to the Networks/Paths Menu and select shortest path.
You should now have the toolbox that allows you to check the paths.
What field would you use to check the
shortest paths between areas????
Check the paths from these zones:
40-44
6-41
13-41
41-46 (both directions)
Did you find an error? If so, what was it?
B) Create a themed plot of speed limits and compare them to the real
world speed limits to make sure they are right! (insert map of correct
speeds?)
Chapter 2 – Building Your Network
12
Modeling 101
Where are we and where are we going????
Up to now you have:
1) Created geographic linework that is cleaned and attributed with the appropriate data
(including changing the node ID’s to represent TAZ’s & travel times)
2) Created your NETWORK to be used in the rest of the modeling process
3) Created a selection set of your centroids (which represent your TAZ’s)
4) Error checked your network & geographic file for coding errors
Chapter 2 – Building Your Network
13
Chapter 3-Building Minimum Paths
What is a minimum path you ask? It is the route or travel path on the
transportation network that has the lowest “cost” for the traveler. The “cost” can be the
travel time, distance or some monetary value. So basically if you choose to go from
home to the mall would you go the shortest distance route regardless of time or would
you take the quickest route based on time….what you choose is your minimum path
based on some cost that is important to you!
Why do we need them? We don’t need them, it’s just a fun step to throw in just to
use your brain a little more…ACTUALLY, we need them to use in two steps in the
model…trip distribution and trip assignment. It helps determine where people will go in
the model and how they get there. In modeling this minimum path value is stored in a
matrix called the impedance matrix ( Since we use travel time in our models, we call it
the travel time matrix). This value is used in trip distribution to determine the zone-tozone travel times. This minimum path value results from the “skim tree” that is
developed. The skim tree is a table that shows all the possible combinations of links or
paths that could be used to go from one zone to another. There is only one minimum
path and that value is the one stored in the travel time matrix. During assignment the
minimum path is used to assign trips on the links in the network. We will learn more
about assignment in later chapters.
How do we create the minimum paths?
Step 1 – Open your geographic file and associated network
*Remember this is what Chapter 1 & 2 were all about!
For the tutorial you should open :
a) the geographic file called: Pilot Mtn BY Final Network.dbd
b) The associated network file called: Pilot Mtn BY Network.net
Step 2 – Unhide the nodes in your file and create a selection set of your centroids by
using the Select by Condition command under the selection menu as you did in the last
section of Chapter 2. Call the selection set CENTROIDS.
Step 3- Make sure the nodes or endpoints layer is the active layer! You should have
something like this:
Active Layer is
the Endpoints
Chapter 4 –Trip Generation
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Modeling 101
Step 4 – Create the minimum paths for your interzonals (these are the trips between
zone A & zone B). This output file will be a matrix of minimum travel times for each
zone to zone pair!
a) Choose Networks/Paths Æ Multiple Paths
You should get the Multiple Shortest Path dialog box.
b) Under the Settings Æ Minimize section you should choose the column that
represents travel time, in this case TTime[min]. TransCAD will minimize time
in both directions from Centroids to Centroids.
Tells you which network
minimum paths are based on!
You can create other skim
tree types like speed or
distance by selecting the
Skims button!
Make sure it says
centroids to centroids so
that only skim trees are
made from one TAZ to
another!!!
Note: You can create other matrices to do some checks on your network by clicking the
skims button on the screen and selecting the skims you want to save. To see the actual
minimum path you must use the minimum path tool located under the Networks/Paths
menu.
c) Click OK
Next the Output File Setting dialog box will come up. Make sure the file will be saved in
the proper directory. If not, go under Look In: and choose the proper directory to save
the matrix. Name the matrix Pilot Mtn BY shortest path. Choose SAVE
Chapter 3 – Building Minimum Paths
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Modeling 101
You should get some flashes….and then this screen showing your minimum path matrix:
Size of matrix is
shown here
This matrix is your interzonals only!!! You will see that from Zone 1 to Zone 1 you have
0.00 as the travel time and that is because the INTRAZONAL time has not been
calculated yet! That is the next step but before we move on make sure you have the
correct size matrix (it should be equal to 46 by 46 for Pilot Mountain)! Remember this
number INCLUDES your externals!
Step 5 – Calculate the Intrazonal Travel Times.
The intrazonal travel time is found in the diagonal of the travel time matrix and
represents the travel time for trips that are made within the zone. For example, if you left
your house and drove up the street to the gas station a block away to get a drink…you
stayed in the zone during your trip, so therefore it was an intrazonal trip. You don’t want
all of the trips in your zone to leave the centroid and get onto the network so you must
calculate the time so that some of the short trips remain in the zone.
The intrazonal trips are important because they directly affect the volumes on the
network:
• The higher the percentage of intrazonal trips the lower the volumes on the
network.
• The lower the percentage of intrazonal trips the higher the volumes on the
network.
The number of intrazonal trips can be adjusted by adjusting the intrazonal travel times in
the trip distribution process.
Chapter 3 – Building Minimum Paths
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Modeling 101
What are these numbers for and how are they calculated?
The way we typically calculate intrazonal times is to use the “nearest neighbor”
technique. We are basically saying that the intrazonal time for Zone A is equal to half the
average time it takes to get to the closest adjacent zones. We generally look at the three
closest/adjacent zones in order to determine the travel time.
In the example below on how to calculate Zone A’s intrazonal time, we would look at
zones B, C, & D not E. The times from those zones would be averaged. The average
time then gets reduced by 50% and the number that is produced is the intrazonal travel
time for the zone you are looking at. Below is the sample calculation for Zone A.
Intrazonal Calculation Example
Zone
B
2 min
Zone
D
4 min
Zone
A
5 min
10 min
Zone
C
Zone
E
Average the travel times for the 3 closest zones:
(Zone D + Zone B + Zone C) Î 4+2+5 = 11 = 3.7
3
3
3
Then reduce the travel time by 50% so:
3.7*50% = 1.85 minutes
This is the intrazonal travel time for ZONE A!!!!
Chapter 3 – Building Minimum Paths
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Modeling 101
How do we calculate it in TransCAD:
Choose
PlanningÆ Planning UtilitiesÆ Intrazonal Travel Times
A dialog box will appear (like shown above)and you will need to fill in the values for
“Factor Values By” and “Adjacent Zones”.
These are described below.
Factor Values By – This is the factor that is applied to the average of the travel
time values from a given zone to all of its adjacent zones (as specified by the
user). The typical value for this factor is 0.5.
Adjacent Zone – This is the user-specified number of adjacent zones to be used
in the calculation of the average travel time. The program determines which
zones are the closest; the user is only required to input the number of zones to be
considered. A typical value ranges from 3 to 5 zones.
The matrix you are trying to calculate the intrazonal impedance for is the shortest path travel time matrix.
Once you have filled out the dialog box like on the previous page click OK and your
travel times for your zones will show up in your matrix now!
You now have your minimum path matrix for your network.
You should do some error checking to make sure your paths make sense!
If I go from Zone A to Zone B does it take me 10 minutes or 20 minutes
and does the matrix compare to my experience in the field!
Check maximum & minimum times. Use the marginals tool in TransCAD to check the min
& max for the matrix. HOW? Right click in the matrix and select properties.
Chapter 3 – Building Minimum Paths
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Modeling 101
You will get a new dialog box like this:
You can choose different things
to look at for error checking!
You should look at these values so you will know what your maximum
time is through your network. This is important to know because when
you develop your friction factors later you will need to make sure they
go out far enough to enable your trips to be distributed correctly!
IMPORTANT NOTE: If you edit your geographic file by adding new lines, editing
existing lines, or changing a speed on a link you MUST RESPECIFY your network,
calculate your travel times and redo your minimum path matrix!!!!!!
Chapter 3 – Building Minimum Paths
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Modeling 101
Tutorial assignment:
1) Recreate the minimum path steps and then answer these questions:
2) What is the maximum travel time for Pilot Mountain based on the minimum path
matrix?
3) What is the minimum travel time for Pilot Mountain based on the minimum path
matrix?
4) How did you get those values?
5) What is the travel time to go from Zone 40 to Zone 5?
6) What steps would I have to redo if I :
a)
b)
c)
Added a new zone to my linework?
Changed the travel time/speed/capacity on a link?
Changed the name of a road in the dataview?
Chapter 3 – Building Minimum Paths
20
Chapter 4 -Trip Generation
Home
What is Trip Generation???
Trip Generation is the first step in the actual 4 step
modeling process. It is where we determine HOW
MANY TRIPS will be made in each one of our zones
(TAZ’s). If you cannot accurately predict how many trips are being made in your area,
then the rest of the process is not going to be accurate and it will be hard to get the travel
demand model to perform at its highest level!
There are different ways to determine how the number of trips are calculated. The basic
tools are:
Cross-Classification : Uses the population (and associated data) in an area to determine
the trip making for each classification type (ie…one worker HH with 2 cars). A lookup
table of values is created and used to estimate the trips for each TAZ.
Regression: The use of a standard regression equation is used to calculate either P’s or
A’s for each TAZ. The equations are based on borrowed or estimated coefficients & input
variables.
Discrete Choice: Using individual level data to estimate the probability of a household to
make a trip.
All of these methods use land use data in some fashion to estimate the trips for each TAZ.
Some use persons/household and auto ownership, while others just use more aggregate
data like housing and employment totals broken into categories. Typically, in our
estimation of trips for the model, the housing unit determines the number of trips
produced (where the decision to make trips originated) and the employment determines
how many trips are attracted (why you went to this destination).
For example: when you woke up this morning you had to decide to make a trip (or
hit the snooze bar). You left your house and came to work. You produced a trip from
your house and were attracted to work. This is the standard rationale of thinking for the
trip generation step….trips are produced and attracted….so you must determine how
many of both!!
Therefore, in order to complete the trip generation step of the modeling process there are 3
basic steps that must be accomplished:
Chapter 4 –Trip Generation
21
Work
Modeling 101
1)
2)
3)
Determine the number of trips produced by each zone
Determine the number of trips attracted by each zone
A balancing of the total attractions and productions
You know why we need #1 & #2 but why #3???????? Any ideas????
Ok times up….similar in concept to what one of those scientists discovered a long time
ago in the Law of the Conservation of Energy….you cannot create or destroy energy…you
cannot have more productions than you do attractions. They must be equal. We can’t just
get rid of the extra attractions or productions so our Trip Gen Program goes through and
adjusts them by moving them around through a weighting process.
Once you complete those 3 steps you pass the test and can move onto Trip
Distribution…the next step.
The basic concept of trip generation is to take Landuse data and
convert it into productions/attractions.
It is important to understand the basic concepts of Trip Generation so that the steps of
How to Do Trip Generation the SWP Way are easier to understand and perform. The rest
of this section discusses step by step how we typically do Trip Generation in SWP. The
data, variables needed, concepts behind our method, the accuracy checks and the
programs you should use are all covered in here.
Section 1 – Data Needs
This is one of the important parts of trip generation because without data it is kind of hard
to determine the trips in your area. Housing, employment and commercial vehicle data
are the three basic data elements that are needed for your travel demand model (at least for
a majority of the models we will build in SWP!)
Housing- Typically the housing data is broken down into the 5 housing rating classes
that represent the trip making characteristics of the persons in the household. By placing
all of your households into one of these categories you are
determining the trip making ability of each household and in turn,
the total number of trips produced in each zone! Our housing
classification system is a surrogate for other trip making
characteristics such as person/HH, income, and auto ownership.
This classification process results in easier variables to collect and
forecast than some of the other trip generation models. Here’s an example of what it
looks like:
Zone #
1
2
Excellent HH’s Above Ave HH’s
20
0
10
52
Chapter 4 – Trip Generation
Ave HH’s
5
41
Below Ave HH’s Poor HH’s
2
4
20
40
22
Modeling 101
This data is entered into the SWP-Trip Gen software program on a zone by zone basis.
The steps for using the program are discussed later in this section.
NOTE: Although we usually think of housing as the producing end of the trip, it also
attracts trips as well. This is accounted for in the trip generation program we use.
Employment – Employment is also a necessary data element and is classified by using
SIC codes or Standard Industrial Classification Codes. There are five basic categories for
employment:
Classification Group
Industry
Retail Employment
Highway Retail
Office
Service
Examples
Factories, Farms
Target, Walmart, grocery store
Gas Station, Fast Food
Lawyers, Real Estate, Insurance
Doctors,Hotels, Automotive Repair
SIC Code #’s included
1-49
50-54, 56,57,59
55,58
60-67,91-97
70-76,78-89,99
Each type of business attracts a different number of trips per employee because of the
function it serves. For example:
Burger World has 25 Employees
Wilma’s WebDesign has 25 Employees
Each business has the same amount of employees but they serve different functions and
therefore attract a different amount of trips. Since Burger World is fast food, it is more
likely people will stop by while out on the town because they are in a hurry, it attracts a
lot of trips. The web design office attracts less trips because typically only the employees
and a few customers may come to this location during a typical day. The web design
office is not an attraction that will get drive by trips, quick stop ins, etc, therefore it
generates less trips. Even though they have the same amount of employees…. Burger
World would attract far more people on an average day!
The information for each business is entered into the SWP-Trip Gen program on a zone by
zone basis and is used to calculate the number of attractions for each TAZ.
Chapter 4 – Trip Generation
23
Modeling 101
Commercial Vehicles – This is often a part of the trip generation
process that does not get the correct attention to detail. Commercial
vehicles are an important part of the trip productions in the model.
The number of commercial vehicles has to be counted as part of field
survey and input on a zonal basis. Single Unit trucks, cars, vans and
pickup trucks are all considered commercial vehicles in this
classification. Taxi’s are a separate classification and should be accounted for since they
generate on average 40 trips/day. Some areas do not have extensive taxi systems so this
may not be an important part of your model.
Well we have all this data that is described above but where does it
go?
Do we just wave our wand and out comes all the results of Trip
Generation????
NO!!!!!
We Must use the SWP
Trip Gen Program!!!!!
Chapter 4 – Trip Generation
24
Modeling 101
Now that you know some of the basic theory behind Trip Generation and you have collected your data for
your area (ie…housing & employment), you have driven around and know approximately how long it takes
to get to places in your area and what part of the town you have the most trips attracted to ( the only grocery
store or Walmart in a lot of cases!!!).
If you have all those details complete then you are ready to enter the information into the SWP-Trip
Generation program. If you are just using this as part of the tutorial, then just pretend like you have done
the data collection and know the time it takes to travel around Pilot Mtn!! Now let’s get started……..
These things should not be out of your reach after you complete the entire
section
-Understand different data types, production/attraction rates and other parameters
entered into the SWP Trip Generation Program
-Calculate Non-Home Based Secondary Trips
-Perform reasonableness checks on the output from the SWP-Trip Generation Program
-Be confidant that your P’s & A’s make sense and can be used in Trip Distribution
What is SWP-TripGen ?
It is an easy to use computer application with a friendly user interface that will take the
landuse data that you collect and convert it into balanced productions (P’s) and attractions
(A’s)! Basically it tells you how many trips are being produced and attracted in each zone of
your area! The program calculates, on a zonal level, your P’s & A’s for each trip purpose
you define (HBW-home based work, HBO-Home Based Other, NHB-Non-Home Base, etc.).
It balances the total trips attracted to the total trips produced, so that we don’t have mystery
trips with no where to go to!!! The program also saves the data into a format that is
TransCAD ready and gives you the ability to check the output data by viewing the appropriate files. These
files are very important for checking the reasonableness of your output. File names and how to use them
will be covered later in the chapter.
What data do I need to be able to use SWP-TripGen?
Well there are two categories of data you need:
a) Ones you collect such as:
- # of dwelling units by classification
- # of employees by category (you know the 5 types discussed earlier in the chapter)
- # of taxis, trucks, commercial vehicles
- External productions/attractions at your external stations (ADT’s minus your Thru trips)this is your EI & IE trips!
b) Ones you estimate using your engineering judgement (this is not to be confused with a
WAG!)
- Percent of HBW, HBO, NHB and the total internal trip percentage
- Occupancy rates for the DU’s
- Generation rates for each DU classification
- Regression Equations for each trip purpose
As we go through the program we will discuss the items in the estimated data category in order to
help you make better decisions in the Trip Generation step, you know give you some rules of
thumb.
The items in the collect category were discussed in the beginning of the chapter so you should know
what they are and how to arrive at them.
Chapter 4 – Trip Generation
25
Modeling 101
Ok- if you think you have all the collected data then you can move on to using the program.
Quick Installation Step before we can use the program –You only have to do this one time, not every
time you want to use the program!
1) Go to this directory:
S:\TransCAD\Trip Generation Program\Package\
or on the CD go to \Trip Generation Program\Package\
2) You should see a file called SETUP.exe (looks like a computer perhaps?)
3) Double click it to start the installation
4) Click OK on the initial pop-up screen
5) You should see this screen now!
6) Click on the computer button and let it install in the default directory location.
7) Click Ok/Continue until the program has completed the installation
The program is now installed and ready for use!!!
Let’s Begin ……
Step 1- Open the SWP-TripGen program by going to the start menu and selecting it under the Programs
menu or you can create a shortcut on your desktop. You should see the program screen pop-up. You should
see this:
You can jam to the music for a while (by clicking on the cat a bunch of times) and prepare
yourself for using the program or just hit START SWP Trip Generation Program.
Chapter 4 – Trip Generation
26
Modeling 101
Now you should see this screen:
As you can see there are a bunch of TABS on this screen. Each one of them has information for you to enter
or to review in order to complete the TripGen program. You do not have to enter your information in any
particular order, just whatever method is easier for you. It is recommended to go in order of the tabs
(numbered in the Application drop down menu) so that you know where you left off entering/editing your
data.
IMPORTANT NOTE: If you already have an old “IDS” file that you would like to use in the new program
instead of entering all the data again you can bring in an existing file that was in IDS format. Follow the
few steps listed below.
If you are beginning from scratch then do not worry about the
step below, just skip to the TAB 1 section !
Opening Existing IDS File
A) Click the Browse Existing IDS File button
Chapter 4 – Trip Generation
27
Modeling 101
B) Find the file you want to open and click OK. ( In this case you open the file PMBYiter1.in)
C) You should see your file open up with a tan colored background similar to this screen:
D) Press F6 or go to the Data menu and select Load Data. You should click OK and then a screen comes
up telling you what data was used/imported (like # of zones & file name). Click OK on this screen and
your background should change colors to yellow now. You can close the yellow box that shows the file
you just imported by clicking the little X and now it has placed your existing data into the Trip
Generation Program. You can now use it!!!!!
The next several sections go through each of the tabs in the program to explain in detail:
- what information goes in each box
- how to arrive at certain data
- the general theory behind the entire Trip Generation Program and variables associated with it
Using the Trip Generation Program – Step By Step (All of the examples use the Pilot Mountain files
for data)
So let’s start with the first tab, which by default shows up on screen when you begin the program.
Step 1 –Description
This is generally the first screen you see when you open the program. It is information about your area!
Step A-
Lines 1 & 2
Step BLine 3
Chapter 4 – Trip Generation
28
Modeling 101
Step A - Go to The Line 1 & Line 2 Boxes
These boxes are for general information about your study area and this input file. Things such as the Name
of the Area, the year of the data (95 Base or 25 future, etc.) , the engineer responsible for this work.
You should enter whatever information helps you to identify this file and the data associated with it !!!!
Enter in your description info and then go to Step B
Step B – Go to Line 3
Here you will see three different boxes:
The first box called NZONES is for you to enter in your total number of zones for your area. Enter in the
total number of zones now! (This INCLUDES your internal zones and external stations!!! ) For the
tutorial we are using Pilot Mountain so what number do we use?
Internal zones are 1-36
External Stations are 37-46
So we have: 36 Zones + 10 External Stations = 46 ZONES
This is the number we enter in the NZONES box (see previous Figure).
Mental Note: Make sure that if for some reason you have created dummy nodes that you include this as
part of your zone total number!!
The second box is called TEXTP. This is the number of external station productions. The EI/IE
productions, as we call them, are the number of trips that are coming from the people traveling on the
facilities at your external station locations. These can be people living outside the area going to one of your
internal zones or from an internal zone going outside the planning area to work, shop, etc. The way to get
this number is to take each one of your external station ADT’s and subtract the thru trips from them (using
Synth output or whatever method you have chosen-See Chapter 5). The number you get is the total external
productions. Once you have all the external productions calculated, then sum them together and this is the
number that goes in the TEXTP box!! As a first run for Trip Generation it is recommended that
you use the through percentages calculated by Synth (meaning, don’t change them), and then adjust them
accordingly before finalizing your P’s & A’s. See Chapter 5 to learn how to get the through percentage!
Here’s the Example for Pilot Mountain(These are the adjusted SYNTH numbers):
Station #
37
38
39
40
41
42
43
44
45
46
Totals=
ADT
2000
700
2500
4100
22200
3700
420
3200
900
23670
63390
Chapter 4 – Trip Generation
Through Through
Trip % Trip Ends
75
1496
33
230
75
1874
90
3688
98
21752
81
3000
25
104
90
2882
30
268
98
23230
58524
EI & IE
Trips
504
470
626
412
448
700
316
318
632
440
4866
29
Modeling 101
The number going in the TEXTP box is the TOTAL # of external productions for your area, in this case
4866.
Here’s the result:
Number of EI-IE
Trips from
previous page!
The third box called NHBSEC is the number of Non-home based trips made by people who do not live in
the planning area (non-residents). Meaning, I live outside of the Pilot Mountain Planning Area. I drive
through an external station to go to my job in downtown. While I am at work, I decide to go get something
to eat for lunch and then back to work. The lunch trip is a Non-Home Based Secondary trip in this case.
Another description is people who are “garaged” outside the planning area but make NHB trips inside the
planning area. There is an actual calculation to arrive at this number but because we are running Trip
Generation for the first time we put a ZERO (0) in this box so that the program will calculate our internal
trip numbers. The NHBS trips are based off of our total external productions and internal trip numbers and
until we run through the program to get those numbers we cannot calculate a value for the NHBS trips.
After you run the program and do the checks on the output you can calculate your NHBS trips and rerun the
program to get your final output. The calculation of the NHBS trips and the performance checks on the
data are covered later in this section, so until then just know why you are using Zero in this box!
on to Step 2 . . .
Chapter 4 – Trip Generation
30
Modeling 101
Step 2 – Trip Percentages
Here is the way the page looks in the SWP-Trip Generation program:
Clicking on the Tips &
Tricks button will give
you the description of
each box on the Trip
Percentages menu!
The trip percentages page helps to define the types of trips that are made in & out of your planning area.
Choosing these percentages should be a carefully thought through process, based on the type of area that
you are modeling. See the discussion that follows for the different trip types and how you should choose
them for your area.
PINT BOX
Typical Range: 80-90
The first box on the menu is the PINT or the Percentage of Internal Trips. These trips are the trips that are
produced by the people inside the planning area AND stay inside the planning area. Of all the trips
produced by your housing units what is the percentage of those that will stay inside the planning area.
This number depends on the opportunity to make trips in the area based on the types of employment and
housing. If you had an area that had a lot of housing but very little employment and most of the people
traveled to other areas to work or shop or play(bedroom type community) then your percent internal would
be lower. Or if most of the people who work at the big industries in town also live in the area, then they
most likely make all their trips inside your planning area. You should know your area well enough to know
the answer to these types of questions. You should also think in terms of all the trips people make on an
average day…..work, school,shopping, etc. and then think about the businesses in your area….do they
provide this type of trip opportunity???
We used 90% for Pilot Mountain.
Chapter 4 – Trip Generation
31
Modeling 101
PHBW Box
This box is for the percentage of Home Based Work trips for your area. It is simply the trip made by people
living inside the planning area from home to work. Typically the percentage ranges from 18-25%
depending upon the area type. If after collecting your data and calculating your employment to population
ratio you got a value over .60, then you could justify raising the HBW percentage to the top of the range.
The HBW purpose should typically be the lowest percentage of trips because people typically only make
that work to home trip twice a day, whereas you might make several other trips to shop or eat from home on
a daily basis. For Pilot Mountain, it is a blue collar area with most of the work trips in industry, so based on
the total number of trips and the higher employment to population ratio we used 23% for HBW.
PHBO Box
This box is for the percentage of Home Based Other trips in your area. It is all of the trips that start from the
home and end up as a shopping, play, entertainment, etc. type of trip. This is usually the HIGHEST
percentage because people make so many other trips in a day and large portion of them are from the home to
some other place besides work. Communities that have small distances/times for commuting from home to
other types of places would have a higher percentage of HBO trips. If it takes someone only 10 minutes to
get home from work then they may go home, change clothes then go to dinner, or to the store. If it took you
30 minutes to get home then you might not go home first before making another trip. The trip would then
not be a HBO trip. These are the things you need to take into consideration when deciding on the
percentage for HBO trips. The opportunity to make these other trips should aide you in your decision as to
the percentage you should use for your area. If you only have one gas station and no Target or Walmart
then there probably aren’t a lot of these types of trips being made….you get the point? Pilot Mountain was
44%.
PNHB Box
This box is for the percentage of Non-Home Based trips in your area. These are trips that originate at some
location other than the home. They are trips to the grocery store on your way home or a trip by the
babysitters to get the kids. Generally these trips are about 1/3 of the total trips in the area. Pilot Mountain
NHB percentage was 33%.
Note: There are other boxes located on this screen and they can be used for OTHER TRIP PURPOSES
if you need them. Shopping and School trip purposes are generally the other types that are used.
Here is a table of values for you to use as a starting/comparison point for your study. Note that these are
larger urban areas and your values would be different based on the population and type of area you are
working with! However, studies have shown that no matter the size if the area the trip percentages seem to
remain relatively stable and within the national average.
Purpose
Triangle Survey*
Triad Survey*
HBW
22%
20%
HBO
46%
49%
NHB
32%
31%
Non-HBW
*Incorporates urban and non-urban households
Charlotte Region*
19%
National
18 - 25%
47 - 58%
18 - 28%
81%
You are now finished with Step 2 -Trip Purposes….
only six more tabs until you are done!!!!!!!
Chapter 4 – Trip Generation
32
Modeling 101
Step 3 – Occupancy Rates
Here’s an interesting little tab! It is the occupancy rates for the housing in your area. Logically you would
have a different occupancy rate for each type of house in your area because the characteristics for those
houses differ. The occupancy would effect the number of trips made by the household if you were doing a
cross-classification trip generation model. However, in our SWP Trip Generation program we set our
generation rates manually and only have one value for each housing class ( as you will see on Tab 4-the next
section). Therefore, this occupancy rate is only used to determine the population in your area and then use
that value to calculate your employment to population ratio. So what does that mean for you??? What do
you need to worry about for this screen??? You just need the average occupancy rate for your area and enter
it in all the boxes on this screen. You should know your population for your area and you also know the
number of housing units in your area as well. Divide the population by the number of housing units and that
is your occupancy rate! Or you may have gotten a occupancy rate for your area from some other source and
you can use that here instead.
Here’s the screen and the values for Pilot Mountain:
Chapter 4 – Trip Generation
33
Modeling 101
Step 4-generation rates
Generation Rates are the average number of trips produced by each of the housing categories in your area.
There are also rates for taxi’s, trucks and commercial autos. The rates assume this is the total number of
vehicle trips made in one day by all people in that dwelling unit. It is assumed that people who have more
income (ie…better houses in this case) will make more trips.
The standard rates for Statewide Planning have typically been:
Excellent DU’s (EXGR)
Above Average (AAGR)
Average (AVGR)
Below Average (BAGR)
Poor (PRGR)
=
=
=
=
=
12 trips/DU
10 trips/DU
8 trips/DU
6 trips/DU
4 trips/DU
The rates are multiplied by the number of dwelling units, by category, for every zone. The number is the
total trips produced by that zone.
As an example, using the standard generation rates and for ZONE 1 you had the following:
Excellent Houses
Above Average
Average
Below Average
Poor
10
15
7
9
14
Then based on the following equation used for determining the productions in each zone:
Total Productions Zonei = (EXGR)XEX + (AAGR)XAA + (AVGR)XAV + (BAGR)XBA + (PRGR)XPR
Where X = number of DU’s
i = Zone number
Total Productions Zone1 = 12*(10) + 10*(15) + 8*(7) + 6*(9) + 4*(14) = 436
This calculation is performed for every zone using the designated generation rates.
You should determine your trip rates per dwelling unit based on the data that you collect in your area. You
should know the opportunity to make trips in the area, look at the ADT’s in the area to get a feel for the
amount of traffic, and the classification of your dwelling unit data.
If you have a lot of average DU’s and very little poor or excellent then you may need to raise the Average
standard rate to a higher value. This is discussed in more detail in the error checking section of this chapter.
You can use the standard rates as a starting point and then adjust them after you complete the first run of trip
generation. You can also borrow rates from another area if it is similar in characteristics such as housing
distribution, employment type, etc.
Chapter 4 – Trip Generation
34
Modeling 101
Here is a table of values from local surveys. You can see are pretty similar in the trips that are made based
on the housing classification:
Vehicle Trip Production Rates
Housing Classification
1995 Triangle
Triad Survey
Household Survey
Excellent
9.4*
9.3
Above Average
9.4*
9.1
Average
8.3
7.7
Below Average
6.2*
6.3
Poor
6.2*
5.7
All Dwelling Units
7.8
7.4 - 8.0
*Categories had to be combined to achieve a statistically significant sample
National Data [FHWA]
11.2
11.2
8.3
5.4
5.4
7.8
Taxi’s and commercial vehicles(autos & trucks) can add extra trips to the network, especially in urban type
areas. You should collect the data for both commercial vehicles and taxi’s in your area because they will
affect the total trips on your network. The typical rates are
Taxi = 40
Truck = 5.7
Commercial Auto = 4.9
The location of these rates is in the last few columns of the Trip Generation Rates Tab.
Below is the screen to enter in the Generation Rates for Pilot Mountain. There were only 3 housing
classification types in this study, therefore only 3 rates are entered into the program. Note that you can
click on the Tips/Tricks button to determine which column is for which rate!!
Chapter 4 – Trip Generation
35
Modeling 101
Tab 5 – regression equations
The regression equations are equations that describe the relative attractiveness of each type of employment.
How many trips are attracted by each type of employment is key in determining how many trips each zone
attracts and ultimately how they are distributed in the next step of the 4 step process. The equations are
based on the number of employees for each employment type.
A typical equation may look like this:
HBO Attractions = .6XInd + 2.50XRet + 6.36XHwy +1.80Off + 1.90XSer + .50DU
Where: X = Number of Employees & DU = number of dwelling units
It is important to understand that based on this equation, HBO trips would be attracted more to the Highway
Retail category than the Industry because the factor for XHwy is a lot higher than the XInd factor. If you have
things that need to be classified as a special generator (like universities, malls) you can enter in separate
attraction equations for them as well.
These equations can be developed from travel survey data or borrowed from other studies. It is important
to understand that when borrowing equations you should look at the type of city you are borrowing from
to determine if the employment types are similar, bedroom community or not, importance of this area for
retail (is it the only place to shop), population/employment ratio similar. You should use the borrowed equations
as a STARTING POINT for your model development. After running the SWP Trip Gen program you will need to
review the results to make sure your equations make sense (the error checking is discussed at the end of this
chapter).
The SWP Trip Gen program has a tab(located on the left of the page) for each trip purpose. You must
enter the attraction equation for each purpose.
Don’t forget to scroll over
and enter the attraction
for the DU’s.
Tab for each
purpose.
Each one
needs its own
equation!
Chapter 4 – Trip Generation
36
Modeling 101
Step 6 – housing totals
This tab is where all of your housing information is added. You should follow the column headings when
entering the data to assure you have your data in the right columns. Here are the column headings and what
they mean for the housing sheet.
ZONEn
NEXn
NAAn
NAVn
NBAn
NPRn
NOT1n
NOT2n
NOT3n
NOT4n
NTXn
NTRKn
NCAn
Zone number for which data applies
Number of Excellent Dwelling units for ZONEn
Number of Above Average Dwelling units for ZONEn
Number of Average Dwelling units for ZONEn
Number of Below Average Dwelling units for ZONEn
Number of Poor Dwelling units for ZONEn
Number of Other 1 Dwelling units for ZONEn (ie. Dorm Rooms)
Number of Other 2 Dwelling units for ZONEn (ie. Motel Rooms)
Number of Other 3 Dwelling units for ZONEn
Number of Other 4 Dwelling units for ZONEn
Number of Taxis for ZONEn
Number of Trucks for ZONEn
Number of Commercial Autos for ZONEn
Here is the Pilot Mountain Housing Totals by Zone:
Chapter 4 – Trip Generation
37
Modeling 101
Tab 7 – employment totals
This tab is where all of your employment information is added. You should follow the column headings
when entering the data to assure you have your data in the right columns. Here are the column headings and
what they mean for the employment sheet.
ZONEn
NX1n
NX2n
NX3n
NX4n
NX5n
NX6n
NX7n
NX8n
NX9n
NX10n
NX11n
NEXTPn
Zone number for which data applies
Group 1 Employment (typically Industrial Employment)
Group 2 Employment (typically Retail Employment)
Group 3 Employment (typically Highway Retail Employment)
Group 4 Employment (typically Office Employment)
Group 5 Employment (typically Service Employment)
Group 6 Employment (typically Other Employment)
Group 7 Employment (typically Other Employment)
Group 8 Employment (typically Other Employment)
Group 9 Employment (typically Special, ie Shopping Center)
Group 10 Employment (typically Special, ie Strong CBD)
Group 11 Employment (typically Special)
Total External Trip Productions (Station Counts minus Thrus)
IMPORTANT NOTE: Don’t forget to enter in your external station productions at the bottom of your
employment. You should scroll down to the external station numbers and enter in the productions in the
NEXTPn column
Chapter 4 – Trip Generation
38
Modeling 101
Here’s the Example for Pilot Mountain for the external stations:
EI & IE
Through Through
Trip % Trip Ends
Trips
Station #
ADT
37
2000
75
1496
504
38
700
33
230
470
39
2500
75
1874
626
40
4100
90
3688
412
41
22200
98
21752
448
42
3700
81
3000
700
43
420
25
104
316
44
3200
90
2882
318
45
900
30
268
632
46
23670
98
23230
440
Totals=
63390
58524
4866
The last column of numbers (EI & IE trips) are the numbers you enter into the SWP trip generation sheet.
You have now entered in all the data necessary to run the trip generation portion of the model for the
first time. Congratulations!!
Note: If you have changed any of your through trip percents you have to rerun trip generation with
the new productions from the SYNTH calculations!!!!!
All you have to do is go to the tab labeled FINAL STEP. It should look like this:
You can review your trip generation file by clicking on the
button. This will bring up
your text file in notepad so that you can scan through it for possible errors.
Chapter 4 – Trip Generation
39
Modeling 101
It will look similar to this:
Click here to close
the input text file!
When you are done reviewing the file you can close it by clicking the X in the corner.
In order to prepare your file for running in the trip generation program you must click on the
button. You should see the Run button
become active at this point.
After
Clicking the
FINISH
button you
click on the
RUN button
to calculate
the P’s & A’s
for your area
Click Here to
Prepare the Trip
Gen File
Click the RUN button and you will see the IDS screen flash briefly on your screen. You have now run the
Trip Generation portion of the model. However, you have done it with ZERO (0) NHB Secondary trips.
We now can start the error/reasonableness checking to make sure we have the correct productions and
attractions in each zone, that our regression equations are producing the correct results, and the percentages
of trip types are accurate.
Chapter 4 – Trip Generation
40
Modeling 101
On to Reasonableness Checking………………..
Trip Generation Reasonableness Checking
You are asking yourself……
What are all the
files it produces?
What do I
look at in
each file?
Where are they
located?
How do I
know if I have
the right P’s
and A’s???
Let’s start with the output files and what’s in them
You will see on the page that there are a bunch of output files you may look at:
In order to view the files you must highlight the file you want to see and click View
Output Files
The output file will be opened as a text file for you to review the results.
Chapter 4 – Trip Generation
41
Modeling 101
Sorting Through the Files
Socio-economic File - This file displays the number of dwelling units by
classification. You should check the totals for each one of your classifications and
make sure they match the field collected data that you have!!! If you have
commercial vehicle and taxi information it will be located in this file as well.
Employment File – This file displays the number of employees by type in your
area. It should match the data from your field survey. You should review the totals
for each type to assure that the data was entered properly.
Population File – This file shows the number of persons in each zone based on
your dwelling unit inventory. It takes the occupancy rate you enter and uses it to calculate the total number
of people in each housing classification for all of your zones. You should perhaps use another source of
data to check against this population result to assure it is reasonable. The population total shown in the
lower right of this file should closely match the total population you have determined for your study area.
Internal Trip File – This file displays the trips produced internally (you know by the houses!!) in your
study area. You can look at each zone and see how many trips are being produced. It is a good way to see
an error jump out at you…. like if one zone has 2000 trips being produced but there are only 25 dwelling
units in it, then maybe there is an error in your data somewhere.
Calculations Trip File – This is the BANG for YOUR Bucks FILE!!!!!
This file gives the results of the unbalanced P’s & A’s, the checks on your rates & average trips, shows you
the trips produced for each trip type and displays the totals for the balancing of the P’s & A’s. The real
ERROR Checking for Trip Generation comes from an understanding and review of this file!
P-A’s Trip File – This is the text file output of your balanced P’s & A’s from the Trip Generation
Program. It has the attractions and productions for each type of housing, employment for each zone in the
study area. This file is formatted and used in TransCAD to perform the final balance on your P’s & A’s.
This files are located in this directory:
C:\Program Files\SWP Trip Generation Program
If you would like to save your interations into another directory you can copy & paste them and rename
them as you see appropriate.
Reminder: If you make changes to this file on screen then those saved changes are the IDSDAT.in file
located in the directory above. You should change the name and save it in another location so that it
doesn’t get overwritten!!
Chapter 4 – Trip Generation
42
Modeling 101
Reasonableness Checking…..What Should I Look at??
The calculations file is the starting point for your error checking. There are a lot of things to be checked in
this file for reasonableness. You should print out the file and use it to perform your calculations either by
hand or in a spreadsheet where you can keep track of the changes you made.(You can use the file called
TGEN CHECKS.XLS located in the Trip Generation folder to calculate your PA ratios!)
Here’s an example of a spreadsheet to track results. See the Next page for the actual calculation file that
was used to fill out this spreadsheet.
Chapter 4 – Trip Generation
43
Modeling 101
Make sure you know
where these P’s & A’s
come from!!!
They are shown on the
next page with a Star
Final Iteration - adjusted Ext prod to Synth fratared totals, includes NHB2 trips
Trips produced by housing units
Internal of Total
Internal Trips
% HBW
%HBO
%NHB
HBW Trips
HBO Trips
NHB Trips
Total CV Trips
Internal CV Trips
External Station Productions
10893
90%
9804
23%
48%
29%
2255
4706
2843
0
0
4866
Average Generation Rate
Employment/Population Ratio
Check Unbalanced Ps and As
HBW
HBO
NHB
IX
Total
Chapter 4 – Trip Generation
8.52
0.77
Ps
As
2257
4708
2845
4866
14676
P/A
2374
4937
3084
4484
14879
0.95
0.95
0.92
1.09
0.99
44
Modeling 101
IDS CALCULATIONS for Pilot Mountain
PILOT MOUNTAIN, SURRY COUNTY - Trip Generation DataJuly 2002 - Pilot Mountain 95 base(1st Run w NHB2=0.July 2002 - Pilot
Mountain 95 base(1st Run w NHB2=0.
***WARNING*** Your average generation rate of 8.52 seems high.
Average should be between 7-8. PLEASE CHECK.
*** WARNING *** The Generation Rate for Above Average Households
0.00 is too high/low. Normal Range is from 9 - 11.
*** WARNING *** The Generation Rate for Below Average Households
0.00 is too high/low. Normal Range is from 5 - 7.
*** WARNING *** The Generation Rate for Poor Households
6.50 is too high/low. Normal Range is from 3 - 5.
*** WARNING *** The Generation Rate for Taxis
0.00 is too high/low. Normal Range is from 30 - 50.
*** WARNING *** The Generation Rate for Trucks
0.00 is too high/low. Normal Range is from 6 - 9.
*** WARNING *** The Generation Rate for Commercial Autos
0.00 is too high/low. Normal Range is from 6 - 9.
***WARNING*** Your emp/pop ratio of
0.77 is out of range. PLEASE CHECK.
Allowable Ranges for Employee/Population Ratio
EMP/POP
INT OF TOTAL
40-50%
80-85%
50-60%
85-90%
TRIP STATISTICS
---------------------------------------------TRIPS PRODUCED BY THE HOUSING UNITS =
10893
INTERNAL OF TOTAL
HBW OF INTERNAL
HBO OF INTERNAL
NHB OF INTERNAL
=
=
=
=
90.0%
23.0%
48.0%
29.0%
INTERNAL TRIPS =
9804
HBW TRIPS =
2255
HBO TRIPS =
4706
NHB TRIPS =
2843
COMM VEH TRIPS
=
0 INTERNAL =
0
TRIPS PRODUCED BY EXTERNAL STATIONS =
NHB SECONDARY TRIPS =
4866
0
AVERAGE DU GENERATION RATE =
8.52 Trips/DU
(Only include the 5 standard housing classes)
EMPLOYEE/POPULATION RATIO =
0.77
(Total Employees in X1 - X8)
REGRESSION COEFFICIENTS
------------------------------HBW
HBO
NHB
ENT-INT
X1
1.10
0.70
0.60
1.00
X2
1.05
3.30
1.50
1.50
X3
1.05
5.50
4.80
5.10
X4
1.05
1.80
1.25
1.60
X5
1.05
2.10
1.40
1.50
X6
0.00
0.00
0.00
0.00
X7
0.00
0.00
0.00
0.00
X8
0.00
0.00
0.00
0.00
X9
0.00
0.00
0.00
0.00
X10
0.00
0.00
0.00
0.00
X11
0.00
0.00
0.00
0.00
DU S CONST
0.00 0.00
0.80 0.00
0.20 0.00
0.25 0.00
CALCULATED ATTRACTION FACTORS
----------------------------ZN
HBW
HBO
NHB
EX-IN
1
87
182
113
136
2
83
207
121
138
3
21
67
30
36
4
534
430
343
536
5
0
16
4
5
6
16
78
29
38
continues for every zone but not all zones displayed here……
45
0
0
0
0
46
0
0
0
0
------- ------- ------- ------2374
4937
3084
4484
These are the UNBALANCED Attractions that go into your spreadsheet!!!
-- Calculation of Productions and Attractions -----------------------------ZONE
TOTAL
INTERL
HBW
OHB
CV
NHB
NHBsec
TNHB
EXT
HBW
HBO
NHB
EX-INT
TRIPS
TRIPS
PROD
PROD
PROD
PROD
TRIPS
PRODS
PRODS
ATTRA
ATTRA
ATTRA
ATTRA
1
235
212
49
102
0
104
0
104
0
83
174
146
779
2
166
149
34
72
0
112
0
112
0
79
197
156
790
3
311
280
64
134
0
28
0
28
0
20
64
39
206
4
273
246
57
118
0
316
0
316
0
508
410
443
3070
Continues for every zone but not displayed here….
46
0
0
0
0
0
0
0
0
440
0
0
0
0
-------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------10893
9806
2257
4708
0
2845
0
2845
4866
2258
4707
2848
4864
Productions Used in the Spreadsheet
Chapter 4 – Trip Generation
For the NHB productions
you must add the
commercial vehicles and
NHB together for the
total
45
Modeling 101
Look at your calculation file and spreadsheet when going through these steps!
1. Check Average Rate per DU Should be 7-8 trips per DU
Yours is < 7 then increase your generation rates
Yours is >8 then decrease OR know why you can justify it being higher!!!
2.
Trips per capita (you know trips/population)
Take the total trips produced by the housing units (shown in the calc file) and divide out your
population (from your population file)
It should be 3-4 trips/person……usually higher than 3.5….if not then raise or lower your rates
accordingly. Or check your occupancy rate, you might have your population wrong!
3.
Check % by purpose and % internal –
Do they fall into a respectable range based on the conditions in my area? Compared to survey results?
4. Check UNBALANCED P’s & A’s by purpose & for the region. VERY IMPORTANT!!!
Your P/A ratio for each purpose should be in the range of .9 - 1.1
If ratio is off then check the attraction equations and make changes.
Here are the attraction rates for the Triangle Survey to use as a guide:
Vehicle Trip Attraction Rates*
Employment Type
HBW
HBO
NHB
Industry
1.2
0.63
1.1
Retail
1.2
3.4
1.0
Highway Retail
1.2
4.2
4.0
Office
1.2
1.2
1.1
Service
1.2
2.0
1.9
Dwelling Units
0
0.9
0.13
*Rates obtained from 1995 Triangle Household Survey
IX
0.34
0.49
0.28
0.28
0.28
0.33
5. Check for reasonableness of your External to Internal Trips!!!
Do I have more externals than internals?-Should that be the case?
What type of attraction is there in my area? Are people gonna come here just to get
gas(ie..highway retail attraction factor)?
Maybe my through trips are not correct?
These steps might take several iterations!! You should think about the numbers logically and make a
decision that you can understand and rationalize. Don’t try and change 3 or 4 factors at once (like
attraction equations and generation rates simultaneously), because then you will not understand what is
actually happening in the generation process.
Chapter 4 – Trip Generation
46
Modeling 101
Keep in mind that there is no “Cook Book” for what changes to make to your model but just some general
guidance. Each area is so different that knowing your area and using your intuition after looking at the
ranges discussed above, should get you in the right direction.
After you have made the changes you feel necessary and are comfortable with the results from your
reasonableness checks then you can calculate the Non-Home Based Secondary trips(Use NHB2.xls to
perform the calculation of these trips-the spreadsheet is displayed below), enter them into the trip
generation program and finally get the final P’s and A’s from the SWP Trip Generation Program.
CALCULATION OF NON-HOME BASED SECONDARY TRIPS
(NHB2)
Pilot Mountain
Trips produced by External Stations (E-I)
(Line 1)
4866
Trips produced by Housing Units (Ih-Ih and Ih-Eh)
10893
Internal Trips Produced By Housing Units (Ih-Ih)
9804
External Trips produced by Vehicles garaged within the P.A. (Ih-Eh)
(Line 2)
Factor (ranges from 0.3 to 0.6, depending on opportunities to make extra trips)
1089
(Line 3)
Trips within planning area that originated from outside the planning area (NHB2)
The final calculation of the Non-Home Based Secondary Trips within the planning area is performed this
way:
NHBS = (Total External Trips - Total EI Trips inside planning area)*Opportunity factor
OR
NHB Secondary = (Line 1 - Line 2)* (Line 3)
You must re-run the Trip Generation Program in order to get the correct P’s & A’s matrix that
accounts for the NHBS trips!!
Keep in mind that the number of NHBS trips can heavily effect the other steps in the model development
process and you should not overestimate them. NHBS trips are made in similar fashion to NHB trips by
residents in the area…typically the percentages are close.
Chapter 4 – Trip Generation
47
0.30
1133
Chapter 5 - Through Trips
At the end of this chapter you should:
* understand what through trips are
* determine, by using SYNTH, your through (external-external) trips
* check to assure the through trips were balanced correctly
By now we understand that a through trip is someone living outside our planning area who travels on a
series of roadways in our area but never makes a stop inside of our planning area. They travel from one side
of the planning area to the other, thus making a trip through the area. Since external travel can have such an
impact on the traffic characteristics in your area it is important to be able to accurately reflect this travel
pattern.
The best estimate of through trips is to use travel survey data to determine the through percentages on your
routes. If travel survey data is not available then the use of SYNTH, a computer program, might be
necessary. The SYNTH program uses regression equations to estimate the number of through trips at
external stations and also to determine how those trips are distributed between all of the external
stations.(For information on the equations and the research on external station development see Technical
Report #3, Modlin or NCHRP829)
This method is only valid for small urban areas, ie…..population under 50,000. It should be noted that in
areas where the population is less than 8-10,000, the estimated through trips may be incorrect due to the data
that was used when the equations were developed. In any case, it is important to
understand that the through trip percentages that are estimated
from SYNTH should be used as a starting point of reference and
should be adjusted based on your knowledge of the area.
How to use SYNTH since you don’t have travel survey data!
Step 1: Copy the SYNTH directory from the N drive to your computer(or from the CD). The
program will output files into the same directory in which the executable is located so you
should place this directory in a location with the rest of your model files.
Step 2: Begin the program by double clicking on the SYNTH.EXE file. You should see
something like this:
Modeling 101
Once this screen is up you type in the name of your summary output file. Type the entire address for the
location you want to save it. So for the example window on the previous page you would enter:
C:\RESEAR~1\PilotM~1\rhetts~1\tutori~1\thrutr~1\thru.out
Note: If you just type in c:\test.thr then it will place the output file in the directory that the SYNTH
executable file is located.
And just to be safe turn on the caps lock button because the program only accepts capital letters as input for
most of the questions it asks!!!!
Step 3: You can enter in your data from the keyboard directly or you can read a file into the program.
Entering the data by hand(using the keyboard) –Read this section but the tutorial uses a file already
created!
If you choose to enter the data by the keyboard you will have to answer the following questions for each
external station:
1 - Description of the Station
2 - Functional Classification of the Road (Comes from the classification maps)
3 - Average Daily Traffic (ADT) – (Comes from ADT book or from special traffic counts you
have taken)
4 - Percentage of Trucks on the Facility (Enter the percent as a whole number,e.g. ten percent
would be entered as 10 ) – can get from special counts or estimated from the general roadway
characteristics for North Carolina
It will ask you if you want to save the file once you enter in all the information. You should save
this file because if not then you will have to re-enter all of the information again, next time you use the
program!!!!
Chapter 5 – Through Trips
49
Modeling 101
using a File to Read the Station information
You must create this file by hand using something like Excel, Wordpad, Notepad.
There are certain items that must be included in this file in order for it to be read into SYNTH.
Here is was the input file should look like:
Pop of Area , Number of External Stations
Station Description , ADT , % Trucks, Functional Classification
(Repeat Previous Line until Station information is entered)
.
.
.
From Route Station ID , To Route Station ID (Note: this is NOT your station number but the ID assigned
.
sequentially by the SYNTH program)
.
.
0,0
(This designates the end of the file!!)
This is a comma delimited file, meaning there is a comma located between each piece of data. A finished
file should look like this:
2752,10
37-SR 1901(KEY ST),2000,3,L
38-SR 2012(OLD52),700,3,L
39-SR 2048(SHOALS),2500,3,L
40-NC268 WEST ,4100,6,J
41-US 52 NORTH,22200,12,P
42-SR1809(WESTFLD),3700,3,L
43-SR 1837(CARSON),420,3,L
44-NC 268 EAST,3200,6,J
45-SR 1855(OLD52),900,3,L
46-US 52 SOUTH,23670,12,P
1,2
2,1
4,8
8,4
5,10
10,5
0,0
The functional classification code used in SYNTH corresponds to this classification chart:
I = Interstate
P = Principal Arterial
M = Minor Arterial
J = Major Collector
L = Minor Collector / Local Road
Chapter 5 – Through Trips
50
Modeling 101
If you choose the file read option you will get this screen in which you enter the filename you want to load.
For this tutorial use the file PMSYNTH.in (located in the thru trip folder).
You can edit the basic input data on the screen if you see data that needs to be changed. Enter in the new
data as directed by each of the questions (HIT enter if you don’t need to change that piece of data) and then
choose N after you are sure your data is entered correctly. You may have a lot of external stations that will
cause you to use more lines on the screen in order to get the data into the DOS window. Unfortunately,
DOS limits the number of lines displayed so you may not be able to see what is being estimated by the
program for some of your earlier stations. It is recommended that you leave the percentages and run
through the entire SYNTH program and printout your final report which shows the percentages. Then
restart the program and adjust your through percentages after looking at the output from the first run.
Your through trips for each station are now estimated through the use of regression equations and displayed
on screen. You may hand adjust the percentages by choosing Y when it asks if you would like to change the
Through Trip Percentages (PTTDES). If your area is less than 10000 then chances are the through
percentages on your major routes are being underestimated by the program due to the fact that the regression
equations did not have any areas with less than 10,000 when they were developed. You should most likely
increase your percentages in this case.
Note: If you change the through percentage it will not be saved in the text file so you will have to
change it every time you rerun the program. Therefore keep the output files stored logically so that
you know what you used for your variables.
Once you are happy with the through trip percentages on each of your external stations you tell the program
you do not want to change any of the percentages (answer “N”).
Step 4 –Route Continuity
If you are reading from a file then the route continuity should come up on screen and be similar to this:
Chapter 5 – Through Trips
51
Modeling 101
When entering in your route continuity you should think about which routes will serve the best continuous
path for people to make their through trip. Your main routes are/should be entered in as continuous from
both directions (usually the higher functionally classified routes). They do not have to be the same route to
be considered continuous.
One thing that is important to consider for continuity purposes is the geographical location of all of your
external stations. You may have a case where you have two major routes that are almost parallel or exist in
such a way that people do not enter at point A and exit at point B (as shown below). In cases like this, it is
important to make routes continuous to all the other higher functionally classified locations so that a large
portion of the through trips are not assigned to the Road A to Road B combination!!!!
D
B
A
C
In this case you would want to make continuous routes from :
A to D
D to A
A to C
C to A
C to B
B to C
D to B
B to D
Basically you make your routes continuous from Point A to all other locations except for Point B so that less
through trips are assigned to that route. You should only worry about this continuous routes issue on the
routes that are NC routes and higher in most areas!!!
After you have your continuous route information entered you type in “C” and hit enter, then spacebar.
Chapter 5 – Through Trips
52
Modeling 101
The next screen will show you the initial matrix for the through trips. You will see that the total for each
station is given at the bottom of the file and a fratar factor (how much the difference in the matrix total is
compared to the number of through desired at that location)
Choose “C” to continue the program and you will see the first iteration on the matrix appear. You should
look at the factor number to see how close all of your stations are to the actual number of trips you desired.
If the number is equal to or close to 1 then your matrix has estimated the distribution of the through trips at
that station pretty accurately. You should continue to iterate (choosing “C”) until you get most of the
factors as close to 1 as possible. Once your factors converge then you can stop iterating and save the results.
It is not recommended to iterate more than 5-6 times!!
Factors need
to converge
to 1.000
You need to save the file in a form that you can use in TransCAD so when it asks if you want a
TRANPLAN matrix file choose yes and choose the name of the file. (For this case name it mythru.txt)
Chapter 5 – Through Trips
53
Modeling 101
Finally you will be asked for your lowest external station number.
Enter in that number so that your output file has the correct station
to station matrix information. This is very important!!!!
You should review the summary file that is output as a result of the program to see if your matrix makes
sense. If you have gone back and redone your through trips after going through trip generation then you
need to make the necessary changes to the trip generation file (you know the external production totals).
Once you have your output file from Synth you are ready to use it in TransCAD.
Creating the through trip matrix for use in TransCAD
Open up your newly created through trip text file (the one output for Tranplan format, in our case the
Thru2.txt file is the right one-although you just created one called mythru.txt, use the thru2.txt file for
consistency of the tutorial) using EXCEL.
Just click FINISH when the excel import wizard comes up!!!!
After Excel has imported the file DELETE the columns filled with ones (should be columns C & D).
Rename each of the columns something logical like shown below:
Chapter 5 – Through Trips
54
Modeling 101
Now save the file as an Excel spreadsheet(Pilot Mtn Thru.xls) so that you can import it into TransCAD.
Importing the matrix into TransCAD
1) With TransCAD open on your desktop, go to File Open.
2) Select Excel Worksheet as the File Type
3) Find the file you saved your through trip file as and click Open
4) After clicking Open you may get a dialog box (like below) that asks you to select the excel sheet and
most likely it will be the only one in your workbook, but select the right one and choose OK!
5) TransCAD wants to save this excel file as a BIN file so give it a name and click SAVE (pilot mtn
thru.bin).
TransCAD will open the file in a dataview similar to this:
Chapter 5 – Through Trips
55
Modeling 101
6) Once you have the dataview open go to the Matrix menu and choose Import
You should get a dialog box like this:
Choose Next
7) Specify the Row ID & Column ID’s that will make up your matrix. You should have Origin Zone &
Destination Zone specified. Click Next!
8) You now have the final dialog box and it asks you to specify what you want the matrix to represent.
Since we need a through trip matrix we want trips from Zone A to Zone B to be the numbers in our
matrix, so we select Trips as the field to fill the matrix with. CLICK FINISH!
Chapter 5 – Through Trips
56
Modeling 101
9) Save the file as a matrix (which should automatically come up). Since it is your through trip matrix I
recommend naming it something meaningful (Pilot Mtn Base Thru.mtx).
ERROR CHECK: You should check your marginals (the sum of each zone-done by right clicking and
choosing properties) and compare it to what you got as your through trips for each of your stations, just to make
sure you imported and are using the right matrix file!
You should also compare the fratared/balanced synth output totals for each zone to the original numbers you
calculated for your through trips!!! When you enter you EI/IE trips into the Trip Gen program (Steps 3&7 from
Chapter 4) you will have to use the BALANCED ones from synth.
For Example:
Use these numbers
in the Trip Gen
Program
STATION
37-SR 1901(KEY ST)
38-SR 2012(OLD52)
39-SR 2048(SHOALS)
40-NC268 WEST
41-US 52 NORTH
42-SR1809(WESTFLD)
43-SR 1837(CARSON)
44-NC 268 EAST
45-SR 1855(OLD52)
46-US 52 SOUTH
ADT
2000
700
2500
4100
22200
3700
420
3200
900
23670
63390
Chapter 5 – Through Trips
PTTDES Thru Trips
75
1500
33
231
75
1875
90
3690
98
21756
81
2997
25
105
90
2880
30
270
98
23196.6
EI/IE
500
469
625
410
444
703
315
320
630
473
4889
THRU
EXT-INT
SYNTH Adj. After Adj. After
SYNTH
SYNTH
output
748
1496
504
115
230
470
937
1874
626
1844
3688
412
10876
21752
448
1500
3000
700
52
104
316
1441
2882
318
134
268
632
11615
23230
440
29262
58524
4866
57
Modeling 101
BALANCED THROUGH TRIP MATRIX:
----------------------------0
0
4
8
344
0
0
0
1
53
This is the
4
0
0
12
430
balanced matrix
8
1
12
0
852
from SYNTH.
344
53
430
852
0
The column
4
1
7
12
655
totals are the
0
0
0
0
18
SYNTH output
3
1
5
11
536
0
0
0
1
46
totals in the
385
59
479
947 7942
chart above!
4
1
7
12
655
0
0
10
1
810
0
0
0
0
18
0
0
0
0
34
3
1
5
11
536
10
0
0
1
874
COLUMN TOTALS:
-------------748
115
937
1500
52
1441
1844 10876
0
0
0
1
46
1
0
1
0
85
385
59
479
947
7942
810
34
874
85
0
134 11615
Important Notes:
Although this chapter comes after the Trip Generation section in the notebook you
should be aware that you have to start determining your through trips before you run
the trip generation program. It is recommended that you use the Synth program
output data as your first run (no changes of the percent throughs, etc.) and input the
external station data you get from it into the Trip generation program. After running
the Trip Generation program and doing the error/reasonableness checks for your P’s
& A’s for the external trips you should go through the through trip chapter to help you
adjust and finalize your through trip percentages. This could be an iterative process.
You need to remember that if you change your through trip percentages that you will
have to re-run Trip Generation with the new external production numbers.
ExerciseUse PMSYNTH.in file and change the percentages to what is listed on the previous page
under the column PTTDES.
Iterate 5 times and save the file as mythru2.txt
Pull into Excel and create the matrix for use in TransCad.!
Note: When you perform the calculations in Trip Generation or in Assignment make sure
you use the file THRU2.txt so that your results are the same as the ones shown in the tutorial.
Chapter 5 – Through Trips
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Modeling 101
Chapter 5 – Through Trips
59
Chapter 6 - Trip Distribution
Just to refresh ourselves, we have determined how many
trips are going to be produced in our area, you know
Chapter 4-Trip Generation. Do you recall how? We used
the Trip Generation program!
We also determined what our through trips (the external to
external trips) were going to be using the Synth program.
So now what?
Trip Distribution is the next step we must perform on our way to building a successful
travel demand model.
What you should know after completing this chapter:
What trip distribution is?
How to get/use/check friction factors
How to perform reasonableness checks on the Trip Distribution Data
How to convert your P’s & A’s to Origins & Destinations
What is Trip Distribution?
Trip Distribution is the step in the modeling process that determines the trip interchanges
between all the zones! Huh?? Could you say that in more understandable terms…..well
it basically takes the P’s & A’s from Trip Generation and determines how many trips
from Zone A are attracted to Zone B and vice versa.
Or as Mr Roger’s would say here’s a picture of it:
Trip Generation
Trip Distribution
300
100
0
20
0
40
550 P’s
400 A’s
550 P’s
400 A’s
500 P’s
250 A’s
200
300 P’s
700 A’s
150
500 P’s
250 A’s
300 P’s
700 A’s
Modeling 101
Thanks to Sir Isaac Newton we use the concept of gravitational theory. The transportation
derivation of this theory implies that the trips made between zones are directly
proportional to the size of the productions and attractions for those zones and inversely
proportional to the travel time for the zones. The actual theory uses size/mass of bodies
to gets the attractions.
Here’s the Gravity Model equation used in Trip Distribution:
T ij = P i *
A j F ij K ij
n
Σ
(A j F ij K ij )
j=1
Where
Tij
Pi
Aj
Fij
Kij
=
=
=
=
=
Number of trips from zone i to zone j
Number of trip productions in zone i
Number of trip attractions in zone j
Friction factor (represents the spatial separation between zone i & zone j
Optional adjustment factor (fudge factor)-not recommended
Zone
Productions
Attractions
D
E
F
800
0
0
0
600
200
Friction Factor
between D & this zone
---1000
500
Based on the above data the number of trips from zone D to E is:
TDtoE = PD *
AEFDE
Σ (AE*FDE + AF*FDF)
600*1000
= 800 *
= 685 trips
(600*1000 +200*500)
Therefore there should be 115 Trips between zone D & F if you calculate using the
gravity model!!!
Now you know what Trip Distribution is and how it is actually calculated in the
modeling process. AND you are now thankful you don’t have to perform these
calculations by hand like when we didn’t have computers!!!
How do you apply the trip distribution theory to the actual model building in
TransCAD??????
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Preparing the productions, attractions, and friction factors
This step is simply taking the productions and attractions produced by the Trip
Generation Program (Chapter 4) and the friction factors and preparing them to be used in
TransCAD.
Let’s start with our Productions and Attractions!
Open Microsoft Excel. Choose File Æ Open, and find the file folder with your P’s & A’s
text file that was created from the Trip Generation Program in Chapter 4.
The Text Import Wizard will pop up.
Choose Finish and let Excel import all the data.
We now need to manipulate this data to a format suitable for TransCAD.
Delete Column A (with the GPs) and Column C (with the ones in it).
Choose InsertÆRows (The entire spreadsheet should shift down one row)
Highlight Column C and Choose Insert Æ Column
Highlight Column E and Choose Insert Æ Column
Highlight Column G and Choose Insert Æ Column
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Modeling 101
You should now have an empty column between each of the columns that contain your
trip productions. Similar to this:
Empty Columns
Label your column titles with the Trip Purposes that you are using in your model. Your
P’s & A’s for each purpose should be located next to each other as shown below:
Now you need to cut & paste your attractions next to your productions, making sure you
place them in the right column!!! The attractions are at the bottom of the productions and
should be located where your zone numbers begin over again (ie at one probably).
Attractions
start here!
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Once you have gotten all the data in the right columns choose File Æ Save As. Save this
entire worksheet as a .DBF file. Use a name that makes sense to you and is easily
understandable like Base Year PA’s.DBF
In the future the Trip Generation Program, discussed in Chapter 4, will automatically set
up this DBF file for you and import it into TransCAD so that you do not have to complete
this step…..but for now you have to do it the long way!
Once you have created this production/attraction PA file in dbf form open it in TransCad.
You should have something like this:
Although the SWP trip generation program balances your P’s & A’s, when you bring it
into TransCad it must be re-balanced. (TransCad uses real number where the Trip Gen
program uses integers.) How do we do this?
With the dbf file open, go to the planning menu and select balance:
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Modeling 101
You should now have the balance dialog box open:
You have to add each trip purpose by clicking the Add button, choosing each purpose in
the Vector 1 field and choosing the type of “hold method”. The hold method tells what
we are keeping constant and balancing to. We usually balance to our productions with
the exception of the NHB trip purpose. You should set up the balance dialog box to
resemble the box below. You can also click on the settings button and select Balance BY
PA’s to automatically set it up for you (assuming you used the names that were suggested
for files).
Click OK and the PA’s will be balanced for you.
Now you can proceed with the other steps.
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Modeling 101
Setting up the Friction Factors
The friction factors must be setup in a similar fashion to the P’s & A’s, in order for
TransCAD to read them properly. If you already have a text file that has your friction
factors, then you can import it in a similar fashion as described previously. If you are
starting a new study and must create your own friction factors or are borrowing them
from another study, then you can just create a new table in Excel or in TransCAD
directly. Regardless of where you create your table of friction factors you need to label
the table appropriately. Here is the preferred look of the table:
Label the columns TIME, HBWFF, HBOFF, NHBFF, IEFF
TIME
1
2
3
4
5
6
7
8
9
10
HBWFF
5800
11600
12200
11000
7750
4500
2500
1500
500
100
HBOFF
5900
12250
15600
11600
6200
3500
2200
1000
200
100
NHBFF
7800
16750
13850
9500
6750
4200
1900
800
100
100
IEFF
10500
13700
14200
13000
5000
2500
1600
500
100
100
HBWFF - Home Based Work Friction Factors.
HBOFF - Home Base Other Friction Factor
NHBFF - Non Home Based Friction Factor
IEFF - Internal/External Friction Factor
Once you have setup the friction factor table you need to save it as a DBF file so that you
can import it into TransCAD as part of Trip Distribution.
IMPORTANT NOTE: Developing & Determining Friction Factors is an addendum
section located at the back of the manual so that you can understand the concept behind
friction factors and how to use them.
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Modeling 101
Applying the Gravity Model
Since you understand the theory and concepts behind the Gravity Model (at least we hope
so from earlier in this chapter), we now need to tell TransCAD on how to find these files
and how to use them. We have the productions, attractions, and friction factors, and we
need to apply the gravity model to generate the production-attraction matrix. The final
output of the gravity model is a zone to zone trip matrix not a PA matrix.
Before starting this step make sure the two databases, the friction factors and the PA’s,
are open and on the screen (if you do not have these opened on screen these steps will not
work correctly). You also will need the minimum path matrix you created using your
network in Chapter 3. Which for Pilot Mountain was called shortest path BY.mtx.
Step 1: Choose Planning Æ Trip Distribution Æ Gravity Application to get the Gravity
Application Dialog box to come up.
Shortest Path
Matrix,
P’s & A’s,
Friction Factors
all OPEN
Gravity
Application
Dialog Box
Filling Out the Gravity Application Dialog Box
Step 2: Under "Production-Attraction Data" use the drop down menu to choose your P’s
& A’s file (BY PA’s for Pilot Mountain) .
Step 3: Make sure the General Tab is selected. Under "Purpose Information for ",
highlight the new and type in HBW (which is our first trip purpose Home Based Work).
Then you must tell the program where to find your productions (it should automatically
find the attractions that match it).
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Modeling 101
Step 4: Under constraint type, choose Doubly so that it performs a total balance between
the P’s & A’s. (Remember our Trip Generation program already produces the P’s & A’s
constrained to the productions so there is no need to do it again)
Your screen should look like the following figure.
Select the Trip
Purpose and tell
it which column
to use
Doubly
Constrained
Step 5: Now go to the Friction Factors Tab
Since we built our own friction factor table we will select that option. You could use
other functions here to determine your factors (this will be discussed in the Developing
Friction Factors Addendum mentioned previously).
You will have to tell it where to find the friction factor table and impedance matrix that
you created. (By default it should find it if you have it open!) Here’s what it should look
like:
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Modeling 101
You will have to add ALL of your trip purposes to this dialog box and complete the information for each one
of them.
After you get all the trip purposes in the Gravity Application dialog box your tabs should be similar to
these:
General Tab
Friction Factors Tab
Just a little tidbit: Use the settings button to record your setup for this dialog box after
you complete it one time!!! That way when you come back you don’t have to change all the info
in the dialog box again!!!!
Once you have all the purposes included in the dialog box click OK!!
When the save as dialog box comes up please help yourself and others who might try to
assist you by naming this matrix something logical. It is your PA Internal Trip Table that
you are creating so that might be a good name for it(BY PA internal Trip Table). Use
that name in the LABEL box as well. By putting the name in the label box, when you
have this matrix open you will actually know what you are looking at!!!
See now you
know what
matrix you are
looking at
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Modeling 101
You should make sure your matrices converged (this is the little dialog box that pops up
on screen).
You should also look at the report to make sure you had all the information correct and
that it ran like you thought. It also is a good way to check back and see what information
you entered for the factors and when you last created this matrix. You have to scroll
down to the bottom of the file to see the recent info.
Here’s a sample of what it looks like:
Now you have created your internal trip matrices, one for each purpose.
Checks to Perform:
1) You should check the trips distributed for each purpose and make sure that the total
matches the total that came out of Trip Generation. Use the marginals tool to perform
this check!!
2) Graph the trip distribution for each purpose to see the shape of the curve. Does it
make sense to you? Does it need to be shifted? Use the addendum in the back of the
notebook to help you
3) Use the Rule of Thumb equations as a check
The next step is converting this PA matrix to an OD matrix!
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Modeling 101
Things you should be able to do after completing this section:
Know how & why we convert P’s & A’s to O’s & D’s
How to get a total trip matrix!
Checks to perform to ensure that the trip distribution step is accurate
The conversion of productions and attractions to origins & destinations is a topic that
often gets confusing when being discussed. We know from our previous knowledge (at
least you should from chapter 4) that productions are the home end of the trip and the
attraction end is work, school or shopping. Based on this concept the P’s & A’s for two
zones would look like this:
P
Zone 1
Home
P
A
Zone 2
Work
A
If we tried to use the PA figure to travel on our network then people would leave home
and never return in a sense! We know that in the “real traveling world” we actually start
at home (our origin) and go to work in the morning (our destination) and in the afternoon
we start at work (our origin) and go home(destination). That travel pattern looks like
this:
O
Zone 1
Home
D
D
Zone 2
Work
O
Since we have built (earlier in this section) a matrix containing P’s & A’s, we know it
functions like the first figure on this page. We need it to function like the second figure
so that the actual directions of the travel patterns are realistic and represent what occurs
on our roadways. This conversion is called???????? You guessed it….the PA to OD
conversion!!!!
Chapter 6 – Trip Distribution
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Modeling 101
Mathematical Process behind the PA to OD conversion:
You don’t have to be able to recite how this conversion occurs but it is important that you
are at least exposed to the “how to” in case you are asked someday (hint hint!). In words,
here’s how the conversion occurs. The PA matrix is added to the transpose of the PA
matrix (added to the transpose of itself) and then that resulting matrix is divided in half to
produce the OD matrix! The transpose is basically a mirror image of the matrix (like if
you saw it backwards) or if you flipped the matrix over by rotating it along the
intrazonals!!!
Scratching your head??????????
Here’s the visual of it!
Here’s Our Original Matrix:
Zone 1
Zone 2
Zone 3
11
12
13
Zone 1
21
22
23
Zone 2
31
32
33
Zone 3
The numbers inside the matrix represent the FROM/TO POSITION in that matrix…ie 12
is Zone 1 to Zone 2
Here’s what our PA matrix with numbers would look like(for three zones in Pilot
Mountain):
Zone 1
Zone 2
Zone 3
1.02
.98
.26
Zone 1
.77
.74
.20
Zone 2
1.57
1.51
.40
Zone 3
Here’s the concept behind the transpose of a matrix:
Zone 1
Zone 2
Zone 3
11
*21
31
Zone 1
12
22
32
Zone 2
13
23
33
Zone 3
Basically we have reversed the order of the numbers in the matrix.
*What was from Zone1 to Zone2 now becomes from Zone2 to Zone1 in our matrix!
Here’s the transpose of our Pilot Mountain PA matrix using the concept above:
Zone 1
Zone 2
Zone 3
1.02
.77
Zone 1
????
.98
.74
1.51
Zone 2
.26
.20
.40
Zone 3
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Modeling 101
Once the matrix is transposed then you add the two matrices together
PA Matrix
Zone 1
Zone 2
Zone 3
Transposed PA Matrix
Zone 1
1.02
.77
1.57
Zone 1
Zone 2
Zone 3
Zone 2
.98
.74
1.51
Zone 3
.26
.20
.40
+
Added Trip Matrix
Zone 1
?????
1.75
1.83
Zone 1
Zone 2
Zone 3
Zone 1
1.02
.98
.26
Zone 2
1.75
1.48
1.71
Zone 2
.77
.74
.20
Zone 3
1.57
1.51
.40
Zone 3
1.83
1.71
.80
Now you divide this matrix by 2 (because the matrix currently has twice the number of
actual trips) and the result of this division is your final Origin/Destination Matrix!!
Final O/D Matrix
Zone 1
Zone 2
Zone 3
Zone 1
1.02
.875
.915
Zone 2
.875
74
.855
Zone 3
.915
.855
.40
This series of calculations is performed for each trip purpose so that you
can get an OD matrix for each purpose in your model!!!
Now you are thankful you don’t have to do that by hand and that TransCAD does it for
you!!!! So how do you do this in TransCAD?????????
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Modeling 101
Converting the Matrices in TransCAD
Step 1: Open your internal PA trip matrix (for Pilot Mountain it is: BY internal trip
table.mtx)
It should look like this:
Step 2 : Open the convert PA to OD dialog box
Go to Planning Menu Æ select P-A to O-D
You should get this dialog box on your screen:
Chapter 6 – Trip Distribution
74
Modeling 101
Step 3: Changing the options on the dialog box
*Make sure you are using the right P-A matrix file
*Uncheck the report each hour separately button.
*Then select, by checking, the Use Matrix HBW button
You will have to select the Use Matrix button for EACH PURPOSE
Your screen should look similar to this:
Check PA
matrix name
Uncheck the report each
hour separately button
Make sure you select
which matrix to use
for each purpose
Step 4: When you have completed the changes in the dialog box click OK and save
the file name as something logical: like Æ BY OD internal trip table
You should now get your Internal OD trip table, by purpose, on the screen.
Look at each
trip purpose
by selecting
here
Chapter 6 – Trip Distribution
75
Modeling 101
You should check your total trips for each matrix again to make sure you performed the
right steps! Check using the marginals and compare it to your PA totals!
Is this the trip table you will use to assign the traffic onto your network???????????
NOPE!!!!!
Remember, this was our INTERNAL trips so we must add the external trips (through) to
this table to get our TOTAL TRIP TABLE!
In order to produce your Total Trip Table you should have your Internal OD table (the
one you just created) and your through trip table open in TransCAD. (Use Pilot Mtn thru
trip table.mtx)
Step 5: Combine the Matrices
Go to Matrix Æ Combine, you should get this dialog box
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Modeling 101
You want to select both the Thru trip table and the internal OD table as the two matrices
to combine. Keep all the rows and columns! Selecting keep only overlapping, will
delete the OD pairs of the columns that are not the same zone numbers, so don’t do that!
Give the file a name, like BY TOTAL TRIP TABLE and you have your trip table that
you can load onto your network after you check it for reasonableness!
Chapter 6 – Trip Distribution
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Model 101
Chapter 7 - Trip Assignment
We have now decided the “trip interchanges” from each zone
to all the other zones….ie distribution. What is left?
The path they will take on screen, loading them on the links
of the network.
What you should know after completing this chapter:
What are the different methods of assigning traffic?
How to use TransCAD to perform assignment
Checks to do on assignment
Traffic Assignment Methods
Highway assignment models load the vehicle trips onto the highway network using a
range of path-building algorithms, and typically iterate each assignment to account for
congestion on the system.
There are two path-building algorithms in use: all-or-nothing (which is the foundation of
several other methods) and stochastic. The all-or-nothing algorithm assigns all of the
trips to the minimum path. The stochastic algorithm estimates a probability that a trip
will take the minimum path or some other “efficient” path, and assigns proportions of the
total trips to various paths based upon the estimated probabilities.
Below are the different assignment methods we would typically use in Statewide
Planning:
All-or-nothing is the simplest form of assignment and is best used to determine
minimum demand paths. It should only be used for small networks with no
base year or future year congestion. This method assigns all vehicle trips
between two zones in a trip table to the links in the highway network
comprising a single minimum time path.
Capacity Restraint assignment changes the impedance of a link as traffic on that
link increases. It attempts to represent the impact that congestion has on route
choice and therefore traffic assignment. It is based on an iterative procedure.
Equilibrium assignment applies the theory of capacity restraint to trips on the
network until no one can improve their trip impedance by changing paths.
Stochastic uses a user-defined probability that allows a trip to take either the
minimum path or some other “efficient” path based on probability estimates.
These methods are discussed in more detail in following sections.
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Model 101
All or Nothing Assignment
All-or-nothing assignment assigns trips to the highway network using the minimum path
between two zones. Assignment using this technique does not take into account delay
caused by congestion, which reduces speed along the minimum path.
This type of loading is extremely sensitive to variation in speeds or other link
impedances. This sensitivity to speed can be problematic when doing assignment in
areas with competing corridors, with similar speeds, such as a central business district of
a city.
Capacity Restraint Assignment
Capacity Restraint Assignment adjusts travel time based on congestion (the volume-tocapacity ratio) using an iterative procedure. The first iteration is an all-or-nothing
assignment for which the analyst specifies the percentage of the trip table to be loaded.
Using this initial assignment, link speeds are adjusted using a volume delay function.
New minimum paths are then computed and another all-or-nothing assignment is
completed for the next analyst specified percentage of the trip table. This process is
repeated until 100 percent of the trip table has been assigned. With this procedure the
final assignment will vary depending on the number of iterations completed (the number
of subsets that the trip table is divided into.) Common break percentages are 70, 20 and
10 or 40, 30, 20, 10.
Equilibrium Assignment
Another variation on the all-or-nothing assignment is the equilibrium assignment. It is
very difficult to obtain good estimates on congested networks with the standard all-ornothing assignment. And while there is more control with the capacity restraint process
that weighs results from iterations, both of these methods can be very sensitive to small
changes in travel time making the process of arriving at just the right combination of
speeds difficult.
An equilibrium assignment looks a several equally good paths through the network when
assigning trips. This can buffer sensitivities by allowing the assignment to run through
several iterations, thereby allowing a small change in speed to equal a small change in
volume.
Stochastic Assignment
A stochastic assignment method uses a user specified probability estimate that a trip will
take the minimum path or some other “efficient” path, and assigns proportions of the total
trips to various paths based upon the estimated probabilities. This assignment captures
travel behavior more effectively than using the all-or-nothing assignment technique
alone.
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Model 101
So How do I do it in TransCAD????????
To begin you MUST have these files open:
1) Your line layer with your modeled roads on it (Pilot Mtn BY Final Network.dbd)
2) The network you created from the linework (Base Pilot Mtn.net)
3) The FINAL OD MATRIX (BY Total Trip Table.mtx)
Make the line layer the working layer!!
1. Choose PlanningÆTraffic Assignment to display the Traffic Assignment dialog
box.
Choose your
assignment
method
QuickSum is all of
the trips types
assigned
ONE WAY
capacity!!!
These constants are
needed for loadings
other than All-or
Nothing
2.
Choose a traffic assignment method from the Method drop-down list.
3.
Choose the travel demand matrix file(usually the Total trip tables) from the
Matrix File drop-down list, and the demand matrix to be assigned from the Matrix
drop-down list(the QuickSum because it is all of the purposes being loaded).
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Model 101
4.
Depending on the method chosen, choose the network fields for time,
capacity(should be one way), and/or preload(generally only for trucks/buses that
do not change their paths).
5.
To use link-specific values for alpha and beta, choose the corresponding network
fields from the Alpha and Beta drop-down lists. ( For now use the default values!)
6.
Depending on the method chosen, make global settings by typing values for
iterations, convergence, alpha, beta, and/or error in the respective edit boxes, and
choose a function from the Function drop-down list.
After you have chosen all the correct values click OK and give your assignment a
meaningful name (ie….all or nothing BY assignment)
We are doing an All-or-Nothing assignment loading first!
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After the fireworks you should now see a dataview on your screen that has joined the
assigned volumes to your base year network data table!!!
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Model 101
(Here are some of the fields in the joined dataview table-just as an example of the data
that exists!)
There are other options that exist in TransCAD that you may feel like using at some point
in your analysis of the transportation system. Here’s the descriptions of those options.
Click on the options button of the Traffic Assignment box to activate the Options Dialog
Box!
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Choose options as follows:
To do this...
Report cold start data -Check the Report Cold Start box and type a value for the cold
start period (in seconds) in the edit box. Used for background info on air quality,
recommended if you are comparing different options or base year to future.
Perform critical (select) link analysis- Choose a table from the Table drop-down list and
choose sets from the scroll list. Sets that are not chosen will only output O-D matrices.
Sets that are chosen will output O-D matrices and additional flow vectors.
Report tabulations of link flows and V/C ratios. Check the Do Tabulation box. This
allows you to look at the link flows by category to do some reasonableness checks.
Skip small values in the assignment-Check the Skip Small Values box and type a
minimum value in the edit box. If you want to not assign volumes under a certain
threshold.
Save link flows in the network-Check the Save Link Flow box and choose a field in
which to store the results from the drop-down list.
Report turning movements-Check the Report Turns checkbox and choose a selection set
to report turning movements.
Specify flow values already present on the links-Check the Warm Start box and choose a
network field that contains the flow values already present on the links.
Create V/C and flow themes of the results-Check the Create Themes box, type a
maximum V/C value in the Max Vac edit box, type an interval value in the Interval Size
edit box, and choose start, end, and intermediate colors from the drop-down lists.
Specify a preload PCE factor- Type a value in the Preload PCE edit box.(HCM 95 or
2000 values are recommended) –Used if you want to take your truck/bus loads and
change them to vehicles to compare alternatives, etc.
Chapter 7- Trip Assignment
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Model 101
Check your results……….
Using the screenline analysis tool you can check the results of your loadings!!
A screenline is an imaginary line on a map, which crosses one or more network links. To
run screenline analysis, you need to specify each screenline that you want analyzed. Any
number of screenlines can be analyzed at the same time.
How do you use the screenline tool in TransCAD??
You can draw and edit screenlines on the map using the tools in the Screenline Editor
toolbox.
1.
Choose PlanningÆPlanning UtilitiesÆScreenline Analysis to display the
Screenline Dataview dialog box.
2.
Choose the screenline definition table that you wish to use as follows:
If you have NOT already created your screenline file then Click New Table and
click OK to display the Store Table In dialog box. Type a file name and click
Save.
If you have created your screenlines already and want to edit them Click Open
Table and click OK to display the Table Stored In dialog box. Choose the table
and click Open.
Or if you already have a dataview open Choose the dataview from the drop-down
list and click OK.
TransCAD displays the Screenline Analysis Settings dialog box.
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Model 101
3.
Make choices as follows.
A)
Choose the dataview that contains the ground counts and assigned flows from the
Line View drop-down list
Choose the fields from the Forward and Reverse Count drop-down lists
Choose the fields from the Forward and Reverse Flow drop-down lists
B)
C)
Click OK. TransCAD displays the Screenline Editor toolbox.
The screenlines are drawn as freehand lines on the map; they are not features in a map
layer. The tools on the toolbox allow you to add, select, delete, and rename screenlines,
and view the links that any screenline intersects.
Here’s what the tools do:
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Model 101
Once you perform the screenline calculations you should get the results of your
assignment in a dataview. There you can look at the ratio to see how your assignment
looks!
Remember the Rule of Thumb- around 10%
If your screenlines are off then you will have to go through the validation/calibration
steps in the final section of this class. This will be covered later.
Other TransCAD stuff for the screenlines
The specifications of screenlines are stored in a screenline definition table. This table
contains all of the information needed to create screenlines on a map and to run
screenline analysis. The screenline definition table is automatically updated when
changes are made to the screenlines using the Screenline Editor.
The screenline definition table contains one or more records for each screenline. Each
record includes the name of the screenline and a longitude and latitude of one of the
points that defines the screenline. The fields are as follows:
Field Contents
ID
SCREENLINE
NAME
LONGITUDE
LATITUDE
A unique ID for each record
TransCAD internal ID for the screenline
Your name for the screenline
The longitude of a point that defines the screenline
The latitude of a point that defines the screenline
The screenline definition table is automatically created, formatted, and updated as you
modify screenlines using the Screenline Editor toolbox. In fact, you never have to
directly look at or modify the screenline definition table to do screenline analysis, since
everything is displayed as freehand lines and can be directly modified on the map using
the Screenline Editor toolbox.
Note: you should not use the regular freehand tools for modifying screenlines. All
manipulation of screenlines should be done from the Screenline Editor toolbox.
Once you have loaded the volumes on your network you should begin the
validation/calibration process. Those steps are included as an addendum(slides) in the
back of the notebook as well.
Chapter 7- Trip Assignment
88

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