Travel Demand Modeling Page
Overview & Purpose
The goal of the Travel Demand Model (TDM) is to forecast the future traffic conditions based on the future land use,
demographic and economic growth. The TDM seems like a black box where data is put in one end and a list of transportation projects comes out from the other. What happens in that black box may not be apparent to those without detailed knowledge of the model and the parameters used to create the model.
This primer seeks to open up the black box and explain how it works and how it is used. The purpose of the TDM is to provide decision makers a picture of the future travel demand in the region and how well the proposed transportation projects will serve the needs of the community. Travel demand modeling helps to answer many of the important questions related to the transportation network such as:
- Which streets will become congested in the future?
- What projects will best improve future mobility for the region?
- How many people can we expect to use the transportation system in the future?
Travel Demand Modeling Process
The Bryan/College Station Metropolitan Planning Organization (BCSMPO) travel demand model follows the traditional three-step process. As the name suggests, this process includes three distinctive steps which help quantify the future travel demands for a region. The three-steps of the traditional travel demand model includes:
STEP 1: Trip Generation (How many trips)
This is the first step of the travel demand modeling process. This step determines the number of trips that will be
produced from and attracted to a traffic analysis zone (TAZ), which are geographical areas primarily defined for
transportation planning purposes. Trips going out from one TAZ are referred as productions, while trips coming into the
TAZ from other zones are referred to as attractions. Thus, each trip constitutes of two trips ends, namely production end
and an attraction end. The socio-economic composition and characteristics of the households are the main
determinants of the number of trips that will be generated from a TAZ. Apart from the trip numbers, this step also
defines each trip by a trip purpose, - work trips, shopping trips, etc. The trip purposes can be further subdivided as
either home based, nonhome based, or other trips types. For example, taking children to school from home would be
characterized as home based school trip, while going to lunch from the office will be characterized as a nonhome based
non-work trip. Thus, the classification of trips as home based or non-home based is dependent on the location of trip
production and attraction ends. In addition to the discussed trip purposes, this step also considers locations in a region
which generate trips that cannot be captured by the above defined trip purposes. For example, Texas A&M home
football games or the Texas Reds Festival, which are major tourist attractions in Brazos County attract a lot of traffic.
The tourist traffic has different characteristics which cannot be classified as either home based or non-home based trips.
These trips are also generated at special trip rates that cannot be captured by the generic trip purposes. Such locations
are identified in the model and are referred to as special generators. For the special generators, trips estimates are
specifically prepared and added to the travel demand model. The final outcome of this step is the number of different
types of trips produced by and attracted to each zone.
STEP 2: Trip Distribution (Where do trips go)
In this step, the origin and destination of the trips are determined. For example, the shopping trips produced in a
residential neighborhood are distributed to other TAZs providing shopping opportunities, and work trips are distributed to
TAZs providing employment opportunities. To distribute the trips to other TAZ’s, one of the most commonly used
methods is the gravity model. The gravity model mimics Newton’s Law of Gravity with travel distance and travel time
being the most commonly used factors to determine attractiveness. For example, you are more likely to make your
grocery shopping trip to a nearby grocery store, instead of the grocery store that is 10 miles away. At the completion of
this step, person trip tables are created for each trip purpose. The trip tables are a matrix of origin to destination (OD
matrix), showing the number of trips between the TAZ’s.
STEP 3: Trip Assignment (What is the route for each trip)
This is the final step in the three-step modeling process. The vehicular trips are assigned to the road network based on
a number of factors, including travel times, congestion, trip length, and travel mode. Upon completion of this step, the
result is a roadway network with traffic volumes assigned to each individual roadway segment. The traffic volumes are
then used to identify the required number of travel lanes and to help determine the location of congested roadways.
Travel Demand Model Integration into the MTP
The travel demand model is a tool and like any other tool, it can only be used for certain purposes and not for others.
Understanding what the model can and cannot do is important for understanding how it has been integrated into the
Metropolitan Transportation Plan (MTP). In general, a regional travel demand model does a good job of forecasting regional travel patterns for planning purposes and a poor job of more operational analysis such as micro-simulation of a specific segment of roadway or a specific intersection.
What the model can do:
- Show the impact of road widening and road additions,
- Analyze the impacts of transportation plans on the regional transportation system,
- Show the impact of new interchanges on traffic patterns,
- Show the impact of large developments,
- Forecast corridor volumes,
- Test alternative land use plans, and/or
- Provide input for air quality conformity.
What the model can’t do:
- Micro-simulations for specific sections of roadway or a specific intersection,
- Forecast turns at intersections,
- Model the impact of intersection or interchange design,
- Model small or local roads very accurately,
- Model bottlenecks,
- Show peak hour volumes or peak hour shifting,
- Show the land use impact of a new road, and/or
- Show impacts of small developments.
The BCSMPO Model will be used to provide information on the effect of proposed transportation improvements. For example, if a road is found to be congested, meaning more cars are on the road than the road can support, the BCSMPO Model can show how much that congestion will be reduced by: widening the road way or providing alternative routes. The outcomes of the improvement strategies can be modeled thereby showing how much each will reduce congestion. This can be used to aid decision makers with the process of choosing and prioritizing future transportation improvements.
If you wish to know more about the new BCSMPO model please feel free to contact the BCSMPO model geek, Bart Benthul at (979) 260-5298 or firstname.lastname@example.org