Navigating the Difference Between Macroscopic and Microscopic Traffic Modeling

Within the past year, the State of Alabama has been transitioning from CUBE Voyager to TransCAD. These Travel Demand Model (TDM) software packages are typically used for statewide models and large urban areas with Metropolitan Planning Organizations (MPOs).  MPOs and states typically develop Long Range Transportation Plans (LRTP) with these software packages; Sain has experience and licenses in both. These models take socio-economic data for hundreds of traffic analysis zones and produce a simulated traffic volume through the “4-step” modeling process: trip generation, trip distribution, mode choice, and trip assignment.

Once the trip generation is validated and the simulated traffic network matches the base year traffic counts for the urban area (or state), the socio-economic data can be forecasted out to 25 years. The model is run again with the projected socio-economic data and can give a snapshot of how the overall network will perform in the future. Alternatives can be developed, and volume-to-capacity (V/C) ratios can be compared to build and no-build scenarios and alternates.

Although complex, the TDM models used for MPO or statewide LRTPs typically do not have signal data, turning movements, or intersection design. For this reason, there are more appropriate microscopic modeling software used for intersection analysis, traffic impact studies, and corridor studies. These utilize the Highway Capacity Manual’s (HCM) equations, including standards based on research, testing, and actual driving conditions specific to intersections and corridors. Two of the microscopic modeling tools used by Sain Associates utilize the HCM formulas and calculations:

  • Synchro & PTV Vistro:  It is best suited for traffic signal analysis and traffic impact studies, as it meets industry standards like the HCM. Its function is to easily optimize and balance networks of signalized and unsignalized intersections and roundabouts.
  • PTV Vissim: It can quantify roadway capacity, delays, queue lengths, travel times, and more. Additionally, it has visualization capabilities beyond the reach of Synchro or Vistro. 

Although Synchro and Vistro are industry standard, PTV Vissim provides more flexibility in analysis. Both packages require a model to be set up with existing conditions/configurations, current traffic counts, and an accepted methodology for forecasting future volumes.

The modeling landscape in Alabama is changing. TransCAD has a lot of bells and whistles to help guide long-range planning decisions on large regional or statewide projects. Technology is improving to include animation on several software packages that can produce future volumes for an intersection, corridors, signal systems, super streets, or roundabouts. These are complex models that are data driven and based on industry standards. Future decisions about local land use decisions are too important to be based on arbitrary assumptions about traffic growth. As the times change, so do the methods of forecasting.