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Travel Demand Model

Contact Info

David S. Heller, P.P., AICP
Program Manager – Systems Performance & Subregional Program
dheller@sjtpo.org

Transportation planning requires the use of sophisticated tools to predict future traffic needs and travel conditions. The specific tool utilized by SJTPO is the South Jersey Travel Demand Model (SJTDM). This computer-based modeling program utilizes numerous inputs including demographic information (e.g. population and employment) and transportation networks (e.g. roads and transit lines) to simulate future conditions. Another major application of the model is to demonstrate that future transportation investments conform with air quality emissions goals established by federal regulations.

In 2015, the model was recalibrated to incorporate the latest available data. The information below provides more detail regarding some of the technical aspects of SJTPO’s travel demand model.

Diagram of road systems in south Jersey
TAZ System

The four-county SJTPO region is divided into 998 traffic analysis zones (TAZs). TAZs are the units used by the SJTDM to generate and distribute trips throughout the region. The SJTDM also extends into neighboring Gloucester and Camden counties for an additional 135 TAZs, while 34 TAZs are external zones used to represent travel at the regional border. TAZ boundaries are loosely based on 2010 Census Block Groups and no TAZ is split by Census Tract boundaries; that is, all TAZs are nested within Census Tract boundaries. For more detail on the TAZ system, see Section 2 of the Model Development Report.

Four-Step Modeling

The SJTDM uses a modified four-step planning method to model travel in the SJTPO region. The four-step method is conducted as follows:

Trip generation: For each TAZ, socioeconomic data (primarily population, households and employment) is used to estimate how many total trips are produced. For more details on trip generation, see Section 5 of the Model Development Report and Section 4 of the Recalibration Report.

Trip distribution: The trips produced by each TAZ are assigned destinations that are distributed among other TAZs. The existing SJTDM currently has seven trip purposes:

  • Home-Based Work
  • Home-Based School
  • Home-Based Shopping
  • Home-Based Other
  • Non-Home-Based Work
  • Non-Home-Based Non-Work
  • Home-Based College

For details on trip distribution, see Section 6 of the Model Development Report and Section 6 of the Recalibration Report.

Mode choice: Each trip is assigned a travel mode. The major modes utilized in the SJTDM are: auto and truck trips, and bus and rail transit trips. For more details on mode choice, see Section 7 of the Model Development Report and Section 6 of the Recalibration Report.

Traffic assignment: Each vehicular trip is assigned a route to take on the highway network. The model also assigns transit trips onto transit rail or bus) lines. For vehicular trips, the result of this step is a loaded highway network for which each highway segment contains traffic volume estimates. For transit trips, the result of this step is a loaded transit network, for which each segment contains bus or rail passenger trips. For details on traffic assignment, see Section 10 of the Model Development Report and Section 7 of the Recalibration Report.

Socioeconomic Data

The SJTDM is calibrated to the base year of 2015. Population and employment data from the Census Bureau is supplied to the model. Indicators of household travel behavior, such as the number of trips each household makes each day, are supplied by the 2014 South Jersey Travel Survey. School enrollment data obtained from the NJ Department of Education was used as additional model input. Population and employment projections through the year 2050 adopted by the SJTPO Policy Board in September 2019 are used to develop future-year model scenarios. For detailed summaries of how the socioeconomic data used in the original 2010 base year of the model was created and developed, see Section 2.3 of the Model Development Report and Section 4.2 of the Recalibration Report.

Highway Network

The highway network used by SJTDM contains all major roadways in the region. Planning-level regional models such as SJTDM typically do not include minor roadways that carry little traffic and are not expected to experience much congestion. Such roads are indirectly represented as centroid connectors, which connect TAZ centroids to the highway network. Key changes made to the highway network during the model update included reducing intrazonal detail in the Atlantic City area and converting the network to the geodatabase format in Cube. All highway construction projects completed through 2015 were added to the network and tolls were updated for proper toll diversion assignment. For details on the highway network, see Section 3 of the Model Development Report and Section 3 of the Recalibration Report.

Traffic Data Sources

A number of traffic data sources were used for model calibration and validation. NJDOT supplies traffic counts for most major regional roadways, including all state highways and many county roadways. Additional traffic counts were supplied by the New Jersey Turnpike Authority, the South Jersey Transportation Authority, and several counties. Traffic counts were assigned to links on the highway network and used to calibrate the model and validate its results. Travel time runs were also used to validate modeled travel speeds. For more details on traffic data sources, see Section 10 of the Model Development Report and Section 7.1 of the Recalibration Report.

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