The ability to provide traffic forecasts on priced roadways has been a key issue for travel demand models. Enhancements have been introduced to better address choices made by drivers about the use of priced roadways, and research has continued in this area.

One issue on which consensus has not yet been reached is where in the modeling process to model the choice of whether to use priced roads. This is always part of route choice (highway assignment), where some trips are assigned to routes using priced roads and some are not. In many models, whether a priced road may be used is also considered as part of the mode choice component. In such cases (“segmented” mode choice models), the auto mode alternatives are duplicated to provide “toll” and “free” alternatives. Those choosing “free” alternatives are restricted from using priced roadways during highway assignment while those choosing “toll” alternatives may (but are not required to) choose paths that include priced roadways.

This question has arisen in the development of many travel models, especially advanced models, such as the InSITE model under development for the Baltimore Metropolitan Council. This presentation discusses the effects of segmenting the mode choice model to include free and toll alternatives and presents an alternative approach to introducing segmentation, which is implemented in the InSITE model.

This approach includes:

• The use of simulated values of time from a distribution in a disaggregate model application;

• Segmentation of trip tables used in aggregate highway assignment by value of time range for both highway assignment and mode choice.

Highway skims would be developed using the implied average value of time for each segment. In mode choice, the skims used for a particular traveler would be those for the value of time range in which the traveler’s simulated value of time falls. Highway assignment would be performed using separate trip tables for each value of time range segment, and skims for the next iteration of the model would be developed for each segment.