Step 3: Mode choice - What travel mode is used for each trip?
The third step of the modeling process is known as mode choice. The current TPB travel model, known as the Version 2.3 Travel Model, uses a nested choice structure as shown below:
The mode choice model includes only "motorized person trips," as defined here. The model includes three auto modes (drive alone, shared ride 2-person, and shared ride 3+person) and four transit modes (commuter rail, all bus, all Metrorail, and combined bus/Metrorail) by three modes of access to transit (park and ride, kiss and ride, and walk). Although the nesting structure does not include explicit branches for specialized transit modes - such as light-rail transit (LRT), bus rapid transit (BRT), and streetcar - the model is designed to deal with these special transit modes (See Chapter 21 of the user’s guide, which can be found on this page, for details).
The mode choice model assumes that these choices are based on the relative availability and attractiveness of each mode. Factors considered in the attractiveness of the mode include:
- Travel time for each mode,
- Travel cost for each mode,
- Accessibility of mass transit,
- Automobile ownership, and
- Proximity to carpool lanes.
The cost variables represent "out of pocket" costs, including transit fares, the price of gasoline, parking, and a mileage rate for driving. Time variables include time spent waiting for transit, time transferring between routes, or time spent parking and “unparking” a car (known as "excess time"). The mode choice factors are arrayed in an equation that estimates the probability of each traveler selecting each mode, given the characteristics of both the mode and the traveler.
Among the most important factors in mode choice are the average parking costs and the time it takes to walk to the final destination from parking spaces or transit stops. Average parking costs for each zone are based on the zonal density of work trip attractions. Thus, heavily used downtown zones have the highest parking costs, and zones with fewer workers per square mile have lower parking costs.
Since access to transit is such an important determinant of whether travelers will choose transit, before the travel model is run, we use a GIS-based procedure to determine the share of each TAZ that is within a short-walk to transit (less than one-half mile) and a long-walk to transit (between on-half mile and one mile).