Route Choice Model

The routes for vehicles without pre-specified paths are generated at each intersection using a route choice model (see Figure 3-2):

where:

p(l|j,t)

probability to choose link l for a vehicle that expects to arrive at node j at time t;

set of outgoing links at node j;

systematic utility of choosing a route with link l as the next link.

perceived travel time on link l at time t;

perceived travel time on the shortest path from node k (the downstream node of link l) to the destination if the vehicle arrives at node k at time t; and

penalty that captures freeway bias.

  
Figure 3-2: Route choice

MITSIM calculates shortest paths from each link to all destination nodes for each driver group. A dynamic shortest path algorithm, which is a modified version of the label correcting algorithm [Ahuja et al.(1993)Ahuja, Magnanti, and Orlin.], is used (see Appendix B for details). The algorithm takes into account: (i) link travel times perceived by a particular driver group; (ii) delays and regulations of turning movements at intersections; and (iii) possible penalties for using certain links (e.g., freeway bias).

Due to the link-based implementation of the route choice model and the fact that a network may have loops, the choice set ( ) should be carefully defined in order to prevent drivers from choosing paths with cycles and circulating in the network. The following tests may be used:

• A link l must satisfy in order to be included in the choice set at an intersection j. In other words, a link must take the vehicle closer to its destination. This constraint is modified from [Dial(1971)].
• The predecessor information (see Appendix B) obtained from the calculation of link-to-node shortest travel times can be used for an explicit test of cyclic path. However, this test can be computationally expensive and unless it is absolutely necessary it should be avoided.