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Innovations in Travel Demand Modeling, Volume 2: Papers (2008)

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Suggested Citation:"T57054 txt_125.pdf." National Academies of Sciences, Engineering, and Medicine. 2008. Innovations in Travel Demand Modeling, Volume 2: Papers. Washington, DC: The National Academies Press. doi: 10.17226/13678.
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lines, in which case there may be no need to run the tran- sit assignment component of the model. THE SLOCOG AND SACOG VISIONING WORKSHOPS The SLOCOG and SACOG workshops’ goals and mod- eling approaches differed substantially. With SLOCOG there was a consensus on future roadway projects but not on land uses; the visioning workshops therefore focused on the type and location of future real estate developments. SLOCOG had a geographic information system (GIS) program— Planning for Community, Energy, Environmental, and Economic Sustainability (PLACE3S)—that enabled it to make quick changes in land uses, and had recently developed a TransCAD model with a fairly short run time (17 min). SACOG, on the other hand, had already achieved a broad consensus on future land uses through its award- winning Blueprint Project. Its new round of workshops was intended to create a consensus on future road and transit projects for its 25-year Metropolitan Transporta- tion Plan (MTP). SACOG had a regional model that operated in a mixture of MINUTP and TP+ scripts with a typical run time of more than 8 h. Table 1 compares the two modeling situations. SLOCOG had the easier modeling task because its original TransCAD model required only a few changes to fit within the run time constraints. The smallest down- town travel analysis zones were consolidated and only a single period (daily) was run. In fact, the run time was fast enough that a team of three modelers was able to service 15 tables of participants, thus eliminating the need to bring in less- skilled staff. Moreover, editing land uses (for SLOCOG) turned out to be easier than editing links (for SACOG) and less likely to cause error. SACOG faced the daunting task of needing to reduce its model’s run time by 97%. It had hoped to achieve this through hardware and software improvements; however, the software upgrade had only a minor impact on run time. Therefore, the model had to be simplified by elim- inating feedback to trip distribution, running fewer assignment iterations, foregoing transit assignment, and limiting roadway assignment to two periods (peak and off- peak) that were then processed in parallel. SACOG also faced difficulties trying to edit networks in a hurry. Attempting to add each project link- by- link was not practicable within the time constraints and would almost certainly have led to coding errors. This task was simplified by preparing a master file containing the existing road and transit networks along with a large number of potential projects— far more than could be included in the MTP. A GIS interface was developed that allowed the links for each proposed project to be modi- fied simultaneously from a drop- down menu. For exam- ple, a proposed 6-lane expressway could be converted into a 4-lane arterial or eliminated by checking the appropriate box on a menu. While the option to edit the attributes of individual links was available, it was rarely used because the participants tended to think in terms of entire projects rather than individual links. In each case certain constraints were placed on partic- ipants to force them to face uncomfortable realities. In the SLOCOG workshops participants were required to accommodate the forecast number of new residents and jobs. The SACOG workshop participants were limited to the programmable portion of the MTP budget, with project costs based on actual estimates (if available) or on average unit costs. Five separate software packages were used in the SLOCOG workshops. The land use data were edited in PLACE3S, which produced an output file readable in Excel. Excel macros were used to reformat the data into a file usable by TransCAD, which produced graphical outputs and tabular indicators. These were combined into a Word file for printing and distribution to the par- ticipants at the originating table and into a PowerPoint presentation for discussion by all the tables. Most of the processing time was spent transferring data from one software package to another. This arrangement was cumbersome and fraught with risk of error, which was only somewhat mitigated by extensive practice prior to the workshops. 125TRAFFIC FORECASTING IN A VISIONING WORKSHOP SETTING TABLE 1 Comparison of Features of SLOCOG and SACOG Workshop Models Feature San Luis Obispo COG Sacramento Area COG Inputs changed Land uses Road and Transit Networks Global constraint on inputs # of new DUs and jobs Total of Project Budgets Modeling software TransCAD Cube/Voyager Processing time (original) 17 min 8 h 35 min Processing time (workshop) 4 min 15 min Modifications made to model Fewer TAZs Fewer periods No feedback to distribution Fewer iterations Fewer periods No transit assignment Some parallel processing Projects as single entities Link to land use software Through Excel Embedded/automatic

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TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 2: Papers includes the papers that were presented at a May 21-23, 2006, conference that examined advances in travel demand modeling, explored the opportunities and the challenges associated with the implementation of advanced travel models, and reviewed the skills and training necessary to apply new modeling techniques. TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 1: Session Summaries is available online.

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