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34 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2 Zonal Accessibility Daily Activity Pattern / Tour Production Log-sums for representative TOD periods Primary Tour Destination Log-sums for all TOD periods Time-of-Day by Half-Tours Highway & best Entire-Tour Mode / Best Transit Sub-mode transit skims Log-sums Stop Frequency by Half-Tours Access by best Stop Location transit sub-mode Transit sub-mode skims Trip Mode Traffic & Transit Assignment FIGURE 1 General structure of the MORPC model system. high-occupancy vehicle; 3, walk to transit; 4, drive to After processing through all tour-level and trip-level transit; 5, nonmotorized; or 6, school bus), and which stages, trip tables are constructed for all modes and tran- one of five transit submodes is chosen for each half tour sit submodes. These tables are assigned to the corre- for walk-to-transit and drive-to-transit tours [1, local sponding highway and transit subnetworks. Loaded bus; 2, express bus; 3, bus rapid transit; 4, light rail tran- networks are skimmed to produce level-of-service attri- sit (LRT); or 5, commuter rail]; and butes necessary for the models. The model system is Stop-frequency model that defines whether there is designed to process through several global iterations, an intermediate stop at each half-tour. Because only one including all (or a chosen subset of) models and network potential stop on each half tour is considered, the model assignments until an equilibrium is reached. at the tour level has only four explicitly modeled alter- Furthermore, several important upward linkages of natives: 1, no stops; 2, outbound stop; 3, inbound stop; the choice models through log sums from the lower-level and 4, stops on both half tours. choices used in upper-level choices are incorporated: Two subsequent models relate to the following trip- Entire-tour bidirectional mode choice log sums for level choices, which are conditional upon the previously the representative time-of-day periods (for example, made tour-level decisions: a.m.p.m. combination for work tours and a.m.midday combination for school tours) are used as variables in Stop-location model that defines a location for the primary tour destination choice models; the reason each stop at the same level of spatial resolution as pri- that only representative mode choice log sums are used mary destination (1,805 zones and three transit-access in the destination choice is that this choice dimension has subzones for each zone). Stop location availability is 1,805 3 5,415 alternatives and is extremely compu- strongly conditional on availability of the chosen tour tationally intensive. mode and transit sub-mode to access the location; and Entire-tour bidirectional mode choice log sums for Trip-mode model that defines mode and transit all time-of-day periods are used as variables in time-of-day submode for each trip on the tour. If there is no stop on choice; because the time-of-day choice model is applied a half tour, the entire half tour is considered one trip and conditionally upon the chosen destination, it is signifi- the chosen mode and transit submode are preserved. If cantly less intensive computationally than destination there is a stop, the half tour is broken into two successive choice, and it is possible to explicitly consider mode choice trips (to and from the stop). log sums for all possible time-of-day combinations.