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D ATA A N D S Y N T H E T I C P O P U L AT I O N S 21 based (NHB). These trip purposes were coded based on There is also a difference in mode share between a lookup table of the 517 possible combinations of pro- trips and tours because of the method used to define the duction place, production activity, attraction place, and primary mode of the tour. Drive alone is a higher prior- attraction activity. The data were then coded into a tour ity than shared rides so that any trip on a driving tour is format. Three codes were developed to support the most a drive-alone trip, with the primary mode recorded as common approaches to tour-based modeling: tour code, drive alone. Transit is a high priority in defining the pri- with trips in the same tour given a common tour identi- mary tour mode, resulting in a 50% higher transit mode fication number; tour mode, which designated the pri- share for tours than for trips. mary travel mode for each tour; and primary destination, A number of conclusions can be drawn from the which designated one of the stops on each tour as the experience with the different surveys in Denver. First, the primary destination. experience with the home interview survey in the devel- A program was developed to group trips into tours. opment of the tour codes suggests that traditional sur- A tour is a sequence of trips starting and ending at home, veys are sufficient for tour-based modeling and that defining a single round-trip. A subtour is a sequence of advanced activity-based surveys are not necessary to trips starting and ending at work defining a single round- develop reasonable tour codes. Second, the experience in trip. To code the tours, the program makes a forward pass Denver suggests that a robust onboard transit survey is through each trip, incrementing the tour identification still needed. Using the brief onboard survey to recruit whenever the traveler departs home. The program tracks transit riders to participate in the home interview survey when the traveler last departed home and last departed provided useful data. The sample size was too small to work for each trip. The program then makes a reverse provide a comprehensive picture of transit use, however. pass through trips, identifying trips where the traveler The results from the Denver survey present a reasonable departed work more recently than he or she departed picture of travel behavior and a realistic first step. The home. These subtours are noted and the model makes one results point out the significant degree of trip chaining more forward pass through the trips, incrementing the and highlight the differences in trip purpose and mode. identification of the subtours and of all subsequent tours. For each tour, one place is designated as the pri- mary destination. In traditional tour-based models, the VALIDATION OF THE ATLANTA REGIONAL primary destination is important because the model COMMISSION POPULATION SYNTHESIZER structure assumes that the activity at this destination controls the behavior of the tour and that other stops are John Bowman and Guy Rousseau scheduled around this activity. The model has 16 differ- ent activity types that are ranked according to duration. John Bowman described the development and the valida- The primary mode of each tour is identified by tion of the population synthesizer included as part of the assigning a priority to the mode of each trip in the fol- new activity-based travel demand model being developed lowing order: school bus, kiss-and-ride, park-and-ride, at the Atlanta Regional Commission (ARC). He described walk to transit, drive alone, share ride 2, share ride 3+, the development of the population synthesizer, the valida- bicycle, and walk. As an example, if any trip on a tour is tion process, and the preliminary results of the validation made by a school bus, that is the primary mode for the process. Volume 2 includes a paper on this topic.2 The fol- tour. Not all trips in a tour have to use the same mode. lowing points were covered in his presentation. The results from the household surveys were used in a trip-based format during the refresh phase, which ARC is developing a new activity-based travel included a partial reestimation and a full recalibration of demand model. It is anticipated that the model will even- the DRCOG trip-based model. The results are also being tually be used for travel forecasts and policy analysis. The used in the update phase of developing the tour-based population synthesizer is the first component of the model travel model. The household survey results show a high to be completed. A population synthesizer acts as a con- level of trip chaining. The data also show differences duit of land use information in a travel demand model. It when coded in trip-based and tour-based formats. For uses information from the census and the land use model example, 17% of trips are HBW trips, while 33% of the and creates a detailed synthetic population consistent with tours are work tours. This difference indicates that work the land use forecasts. A population synthesizer includes a is a key reason for travel even though the number of record for each household in a region and a record for HBW trips is relatively small due to trip chaining. A com- each person in that household. A base-year and back-cast parison of trip purpose to the primary purpose of the validation was conducted on the ARC population synthe- tour for each trip record indicates that only half of all trips on work tours were coded with a HBW purpose, 2 See Bowman, J. L., and G. Rousseau. Validation of Atlanta, Georgia, while the other half were HBNW or NHB trips. Regional Commission Population Synthesizer. Volume 2, pp. 5462.