Background
The Bannock Transportation Planning Organization (BTPO) uses a Travel Demand Model (TDM) to forecast the future traffic conditions based on the future land use, demographic and employment. 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.
Travel Demand Model Process
The BTPO travel demand model follows a modified four-step model. The steps are the different procedures used in the model to predict travel within the planning area.
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 the various traffic analysis zones (TAZ), which are geographical areas primarily defined for
transportation planning purposes.
The socio-economic composition and characteristics of the households determine the number of trips that will be generated from a TAZ. Trip generation also defines the purpose of each trip. by a trip purpose, – work trips, shopping trips, etc. The trip purposes can be further subdivided as
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 the model uses a destination choice model.
The destination choice model predicts the probability of choosing any given TAZ as the trip attraction. The destination choice models use information such as distance, zonal employment, household characteristics, and region
geographic characteristics
Step 3: Mode Choice
BTPO uses a modified Mode Choice model. In traditional mode choice models, the mode (bicycle, vehicle, walk, transit) is determined by the model. THe BTPO uses information from a household survey to the number of non-vehicle trips between zone. That number is removed from the trip table developed in Step 2.
STEP 4: 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, and trip length. 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 Update
BTPO is in the process of updating the travel demand model. The update process involves calibrating and validating the travel demand model to 2020 conditions. Calibration is the process of ensuring the travel characteristics (trip length and travel time) match the 2020 actual conditionals. Validation is the process of comparing the travel demand model predicted traffic volumes to traffic volumes collected on various streets in the region.