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trip models
The stop frequency model assigns to each tour the number of intermediate destinations a person will travel to on each leg of the tour from the origin to tour primary destination and back. The model incorporates the ability for more than one stop in each direction, up to a maximum of 3, for a total of 8 trips per tour (four on each tour leg).
Intermediate stops are not modeled for drive-transit tours because doing so can have unintended consequences because of the difficulty of tracking the location of the vehicle. For example, consider someone who used a park and ride for work and then took transit to an intermediate shopping stop on the way home. Without knowing the vehicle location, it cannot be determined if it is reasonable to allow the person to drive home. Even if the tour were constrained to allow driving only on the first and final trip, the trip home from an intermediate stop may not use the same park and ride where the car was dropped off on the outbound leg, which is usually as close as possible to home because of the impracticality of coding drive access links from every park and ride lot to every zone.
For trip other than the last trip outbound or inbound, assign a purpose based on an observed frequency distribution. The distribution is segmented by tour purpose, tour direction and person type. Work tours are also segmented by departure or arrival time period.
The trip (or stop) location choice model predicts the location of trips (or stops) along the tour other than the primary destination. The stop-location model is structured as a multinomial logit model using a zone attraction size variable and route deviation measure as impedance. The alternatives are sampled from the full set of zones, subject to availability of a zonal attraction size term. The sampling mechanism is also based on accessibility between tour origin and primary destination, and is subject to certain rules based on tour mode.
All destinations are available for auto tour modes, so long as there is a positive size term for the zone. Intermediate stops on walk tours must be within X miles of both the tour origin and primary destination zones. Intermediate stops on bike tours must be within X miles of both the tour origin and primary destination zones. Intermediate stops on walk-transit tours must either be within X miles walking distance of both the tour origin and primary destination, or have transit access to both the tour origin and primary destination. Additionally, only short and long walk zones are available destinations on walk-transit tours.
The intermediate stop location choice model works by cycling through stops on tours. The level-of-service variables (including mode choice logsums) are calculated as the additional utility between the last location and the next known location on the tour. For example, the LOS variable for the first stop on the outbound direction of the tour is based on additional impedance between the tour origin and the tour primary destination. The LOS variable for the next outbound stop is based on the additional impedance between the previous stop and the tour primary destination. Stops on return tour legs work similarly, except that the location of the first stop is a function of the additional impedance between the tour primary destination and the tour origin. The next stop location is based on the additional impedance between the first stop on the return leg and the tour origin, and so on.
After running trip purpose and trip destination separately, the two model can be ran together in an iterative fashion on the remaining failed trips (i.e. trips that cannot be assigned a destination). Each iteration uses new random numbers.
For each trip, assign a departure hour based on an input lookup table of percents by tour purpose, direction (inbound/outbound), tour hour, and trip index.
The tour hour is the tour start hour for outbound trips and the tour end hour for inbound trips. The trip index is the trip sequence on the tour, with up to four trips per half tour
For outbound trips, the trip depart hour must be greater than or equal to the previously selected trip depart hour
For inbound trips, trips are handled in reverse order from the next-to-last trip in the leg back to the first. The tour end hour serves as the anchor time point from which to start assigning trip time periods.
Outbound trips on at-work subtours are assigned the tour depart hour and inbound trips on at-work subtours are assigned the tour end hour.
The assignment of trip depart time is run iteratively up to a max number of iterations since it is possible that the time period selected for an earlier trip in a half-tour makes selection of a later trip time period impossible (or very low probability). Thus, the sampling is re-run until a feasible set of trip time periods is found. If a trip can’t be scheduled after the max iterations, then the trip is assigned the previous trip’s choice (i.e. assumed to happen right after the previous trip) or dropped, as configured by the user. The trip scheduling model does not use mode choice logsums.
Alternatives: Available time periods in the tour window (i.e. tour start and end period). When processing stops on work tours, the available time periods is constrained by the at-work subtour start and end period as well.
The trip mode choice model assigns a travel mode for each trip on a given tour. It operates similarly to the tour mode choice model, but only certain trip modes are available for each tour mode. The correspondence rules are defined according to the following principles:
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Pay trip modes are only available for pay tour modes (for example, drive-alone pay is only available at the trip mode level if drive-alone pay is selected as a tour mode).
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The auto occupancy of the tour mode is determined by the maximum occupancy across all auto trips that make up the tour. Therefore, the auto occupancy for the tour mode is the maximum auto occupancy for any trip on the tour.
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Transit tours can include auto shared-ride trips for particular legs. Therefore, ‘casual carpool’, wherein travelers share a ride to work and take transit back to the tour origin, is explicitly allowed in the tour/trip mode choice model structure.
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The walk mode is allowed for any trip.
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The availability of transit line-haul submodes on transit tours depends on the skimming and tour mode choice hierarchy. Free shared-ride modes are also available in walk-transit tours, albeit with a low probability. Paid shared-ride modes are not allowed on transit tours because no stated preference data is available on the sensitivity of transit riders to automobile value tolls, and no observed data is available to verify the number of people shifting into paid shared-ride trips on transit tours.
The trip mode choice models explanatory variables include household and person variables, level-of-service between the trip origin and destination according to the time period for the tour leg, urban form variables, and alternative-specific constants segmented by tour mode.
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