Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
ibrated against the 1990 MTC survey data for primary destinations by purpose and distribution of trip length frequencies. The results reflect reasonable allocation of destinations among four areas of the city and those des- tinations located outside the city. The estimate of employment that results from the work location model compared with actual employment by neighborhood showed that results were reasonable when compared with estimated values by neighborhood. The biggest dif- ferences were the two neighborhoods in the core busi- ness district, which were underestimating employment, but calibration results also showed that the destinations in the core were within 3% for each tour type and were actually overestimated in these results. The destination choice model was also calibrated by comparing trip length and duration frequency distributions. These results showed reasonable average trip lengths and dura- tions for all tour types. The validation of the intermedi- ate stop choice model was challenging because similar models of destination choice had not included separate validation of the intermediate stop choice component for comparison. The results of this validation test were that both work and other tours were overestimated slightly by the model, while work- based tours were underesti- mated. Additional calibration adjustments to try to rec- oncile these differences were not pursued because further adjustments would have negatively affected the results of the highway assignments by time period. ⢠Mode Choice (Tour- and- Trip) Models: The tour- and- trip mode choice models were calibrated by tour purpose. The calibration results for tour and trip modes showed a close match between estimated and adjusted observed tours and trips by mode and purpose. Initially, estimated transit boardings were discovered to be much higher than observed boardings, particularly for local bus and Muni Metro transit modes; it was concluded that either the transit calibration target values generated from the household survey were too high or the observed transit boardings were low. Because the transit board- ings were calculated annually by Muni, they were held constant, and both the observed and estimated transit shares were adjusted to match boardings better. ⢠Trip Assignment: There were two major modes for assignment validation: highway and transit. These were validated separately by using observed volumes of vehi- cles and passengers on the highway and transit systems, respectively. Assignment validation at the county level was completed by means of aggregated volumes by cor- ridor (identified by screen lines), type of service (facility type, mode, or operator), size (volume group), and period. Speeds and travel times were also used in high- way and transit validations to ensure that these were accurately represented in the models. The highway assignment results were compared for five periods and the average daily results. All targets of highway assign- ment validation were met except for two screen lines and one neighborhood. For transit assignment, all modes were within 5% of the observed transit boardings. How- ever, there were some distinct differences by time of day, with estimated bus boardings significantly greater than observed boardings in the a.m. peak period. Matching the number of a.m. bus boardings within 5% would require a 30% reduction in work transit tours compared with the observed data from the 1990 MTC household survey. An independent estimate of U.S. Census journey- to- work data indicates that the observed transit share of work tours (35%) is reasonable. Therefore, the observed work walkâtransit share was held constant, causing an overestimation of a.m. period local bus trips. COMPARISONS WITH TRIP- BASED MODEL The comparisons of the San Francisco tour- based model with the MTC regional trip- based model showed expected differences in the base year model and some interesting differences in the forecast year model. Because the base year models were both validated to observed data sets, the authors did not see as many dif- ferences in those as in data for the future, when impacts of various forecasts showed different effects in the mod- eling systems. Base Year 1998 SF- CHAMP predicted tours by type rather than trips, so a direct comparison of the home- based work trips was difficult (3). The 1996 MTC survey was used for cali- bration because the number of trips within San Fran- cisco County was very low in the 1990 MTC survey (used to calibrate the MTC trip- based model) due to underreporting of trips that occurred in that survey. The underreporting of trips was not consistent across periods or across trip purposes, which may have influenced model estimation that was based on the 1990 MTC sur- vey. Off- peak periods and work- based and other tours were all underestimated as a result. Trip rates per household were compared by trip pur- pose and showed that trip rates overall were similar, but the trips per household by trip purpose were quite dif- ferent. For example, the model underestimates work and school trips compared with the MTC survey, but this dis- crepancy can be attributed to the surveyâs definition of a trip to work or home as containing all trips to and from work or school. The San Francisco model differentiates between trips to work or school with an intermediate stop from those without an intermediate stop and thus 27THE SAN FRANCISCO MODEL IN PRACTICE
