Project Connect Modeling Process

Evaluating the effect of transportation improvements on the network performance is a complex question, more specifically when it involves multiple modes of transportation (aka, cars and transit). Therefore, it will require it to be evaluated in a modeling platform that accounts for network supply (roads and transit networks), demand (vehicular and transit trips), and all its other corresponding elements at a regional level. Such a platform, commonly known as a travel demand model (TDM), is developed by the Metropolitan Planning Organizations (MPO) and used for consistent regional transportation planning and alternative analysis activities across the region. The CAMPO TDM was used in the I-35 study as required by the FHWA since federal funds are used for this major construction project. This allows for a uniform evaluation between all project alternatives in the region. The CAMPO TDM is developed based on accepted industry practices and can be used for evaluating the different I-35 alternatives and their effect on the roadway and transit networks.

The CAMPO 4-step time-of-day TDM, performs the trip generation, distribution, mode choice, and assignment steps. The trip generation and distribution are typically modeled by using local household surveys. The mode choice typically has a multinomial logit or nested multinomial logit structure estimated based on transit passenger surveys and calibrated to actual base-year transit ridership data. The traffic assignment results are calibrated to base-year field counts. The development of these steps typically follows accepted industry TDM guidelines.

The traffic assignment process follows a generalized cost capacity-constraint equilibrium concept where all paths between all origin-destination pairs are utilized. The transit assignment looks at transit routes’ connectivity, fares, walk/wait times, and transfer fees among other factors to find the best transit path for the transit trips.

The CAMPO TDM’s roadway network was modified to reflect the proposed I-35 roadway alternatives. The approved/funded elements of ProjectConnect changed after the MPO model was approved. Therefore, transit network edits were also needed to reflect the latest CapMetro approved/funded plans for Project Connect.

The TDM was first run with original CAMPO TDM assumptions. The runs were repeated with modified model inputs to make the transit alternatives attractive and to produce ridership forecasts on the Blue and Orange lines that were more in line with CapMetro assumptions (about 120,000 transit riders per day). This ridership matches the currently planned capacity for these routes as well. The adjustments made to the model inputs were as follows:

  • Eliminated the attributes below from the entire transit system:
    • Direct fares and transfer fares.
    • Dwell times.
    • Transfer wait times.
    • Transfer penalty.
  • Reduced the transit attributes below across all routes and modes:
    • Headways (by 90%).
    • Weight of all transit-related time components (to 1.0).
  • Increased the transit attributes below across all transit routes and modes:
    • Walk access/egress speeds (to 10 mph).
    • Maximum access/egress walk time to transit stops.
    • Maximum drive time to park and ride facilities.
    • Maximum transfer time.
    • Maximum wait time.
    • Maximum impedance.
    • Maximum number of transfers.
    • Maximum transit trip time.

The above modifications increased the forecasted system-wide transit ridership by a factor of 2.5 with the goal of utilizing as much transit capacity as realistically possible while producing acceptable ridership forecasts for Blue and Orange lines from the CAMPO model. Hence, overall, this run is a hypothetical Max Transit Ridership scenario just to see what would happen to I-35 traffic under such an extreme transit attractiveness.

The I-35 corridor is forecasted to carry between 280,000 and 320,000 vehicles per day in the year 2045 within the central segment. The above changes in the transit assumptions resulted in a 2.5%-3.0% reduction in the daily I-35 demand. While Project Connect is contributing to an increase in system-wide transit ridership, improving connectivity and transit travel times, and improving overall roadway network congestion, the observed difference in the I-35 volumes is insufficient.