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A P P L I C AT I O N O F A M I C R O S I M U L AT I O N M O D E L 35 · Stop-location log sums (behaviorally interpreted as their primary destinations, and the build scenario is run for density of stop attractions on the way to and from the several iterations, with the inclusion of only mode, stop fre- primary destination) are used as variables in the stop- quency, and stop location choices, as well as assignments. frequency model; these log sums are calculated for each The model output directly used for the UB calculation half tour and take into account stop-location access by relates to the tour-level mode choice statistics only. Tech- the chosen tour mode and transit submode. nically, it is similar to the conventional model output in which trip units are replaced with tour units. The impact The tour-based structure imposes some problems in of other choices (stop frequency, stop location, trip mode the way that model outputs can be compared for the base choice) is taken into account implicitly through the over- and build scenarios. The core complication is that both all iterative equilibration of travel times and cost and trip distribution and mode choice stages are closely inter- upward log sums included into the tour-level mode twined and cannot be fully separated. Indeed, the former choice utilities. single-trip distribution stage is divided into primary-tour The individual-record format can be converted into destination choice and stop-frequencylocation choice. the quasi-aggregate format that corresponds exactly to The former mode choice stage is divided into entire-tour the conventional SUMMIT input. The conversion is mode choice and trip mode choice. These choices are based on the following rules: sequenced in a way that pure trip distribution and mode choice stages cannot be recombined in a simple trip for- · All tour records with identical production zone, mat. Consequently, the basic requirement of a fixed trip attraction zone, socioeconomic market segment, travel table cannot be met without some enforcement in the purpose, and time of day are collapsed into a single (aggre- model chain. However, there are three possible construc- gate) record with the corresponding values for these fields. tive ways to meet the FTA requirements, at least on a · The other fields for the aggregate record are partial basis, and to provide meaningful inputs for UB processed in the following way: calculation that can be processed by SUMMIT. These Person trips are totaled across the aggregated three options are outlined in Table 1. records. Currently, the first (simplest) option has been adopted Fractions of trips that have a walk-to-transit for the MORPC model and implemented programmati- path and drive-to-transit-only, as well as walk-to- cally. According to this approach, a full microsimulation transit and drive-to-transit-only shares, are aver- model (with several global iterations that include all steps) aged across the aggregated records (which makes is run for the base scenario. Then, all tours are fixed with their values fractional). TABLE 1 Three Options for Comparison of Transit Alternatives Model StageFeature Option 1 Option 2 Option 3 Tour generation Fixed Fixed Fixed Tour primary destination Fixed Fixed Fixed Tour time of day Fixed Fixed Fixed Tour mode Variable Variable Variable Stop frequency Variable Fixed Fixed Stop location Variable Variable Fixed Trip mode choice Variable Variable Variable Tour OD tables by TOD periods Fixed Fixed Fixed Trip OD tables by TOD periods Variable Variable but totals are fixed Fixed Report to SUMMIT Simplified tour-level mode Simplified tour-level mode Full tour-level mode choice log sum choice log sum w/o choice log sum w/o with LOS variables reflecting intermediate stops intermediate stops frequency and location of intermedi- ate stops Included components Differences in mode utilities Differences in mode utilities Full differences in entire-tour mode w/o stops w/o stops utilities including LOS variables associated with making stops Ignored components Additional LOS components Additional LOS components associated with making stops; associated with making stops; a certain incomparability of a certain (but less significant) LOS variables for alternatives incomparability of LOS with different number of trips variables for alternatives with the on the tour and different same number of trips on the tour stop locations but different stop locations