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San Francisco County Transportation Authority (SFCTA) Travel Model Peer Review Report

6.0 Panel Discussion and Recommendations

The following text summarizes the panel's discussion on the topics of interest to SFCTA and the audience. The summary of this discussion follows the panel's final presentation back to SFCTA. The discussion begins with some general observations made by the panel on the overall modeling process followed by short and long-term recommendations. Section 6 includes a point-by-point response to the questions the SFCTA posed to the panel at the outset of the meeting. The summary in this section is a consistent summary of the detailed responses in the form of a recommended action plan.

6.1 Observations: General Comments

The panel applauded the overall effort made by SFCTA to systematically develop and calibrate a DTA model, and specifically on the SFCTA's focus on the calibration of the traffic flow model parameters using observed data. They understand that this is a non-trivial endeavor and are not aware of many efforts that have been made in this direction and at such detail. The panel recognizes that this has been a truly collaborative effort with committed staff from SFCTA, the consultant team, and partner agencies which has laid a foundation for further research and application in this area of travel modeling. The panel also commended SFCTA's commitment to the development of an open-source code-base that can help other agencies who would like to develop DTA models in the future. The data driven approach taken by SFCTA towards this project has in the panel's collective experience been known to be a very effective approach, rather than synthesizing inputs or using a default set of parameters. The panel suggested that it would be useful to always keep the applications of this tool in perspective, which would then guide the expansion of its capabilities and sophistication.

6.2 Recommended Shorter-Term Priorities

SFCTA requested that the panel's recommendation focus on what can be achieved in the next few months before the end of the current project. The panel feels that the SFCTA should focus on the following items in the relatively near term:

In addition, a temporal profile at external gateways and in internal zones could be created to avoid a flat or uniform loading profile to the DTA model. Since traffic counts are available, they may be used to create the temporal profile. Alternatively, departure times from the household survey could also be processed to obtain a temporal profile.

Alternatively, a separate facility type for residential low-volume streets could be created. In addition to this, a reaction time factor that includes friction due to pedestrian traffic on such streets could be tested.

In addition to area type and facility type currently being used to classify the various parameters, there is potential to use a third dimension - "intersection type" that would allow for targeted improvements.

6.3 Recommended Longer-Term Priorities

The panel urged SFCTA to focus on the above priorities and to undertake other potentially good ideas over the longer term. These longer-term priorities are:

For freeways, effective length was derived from data collected on local and arterial streets. The panel suggested that SFCTA could try using aerial photos for estimating effective length on freeways.

The panel noted that data on local streets would also be useful since at present, it may not be very clear as to why the predictions from the model are not validating very well against the counts. It is possible that the traffic volumes being simulated on local streets by the DTA model are reasonably accurate but are not being supported by the limited data currently available.

The panel felt that market segmentation of the demand being passed on to the DTA model could be refined further to include an additional dimension based on value of time (VOT). The panel acknowledged that there will possibly be run time implications of this step.

The panel recommends that reliability only be considered after model linkage has been well established at a reasonable level of temporal detail.

The DTA model could eventually be temporally expanded to simulate a full day. The panel felt that it may be better to prefer temporal over spatial detail while increasing the scope and sophistication of the model. A fully disaggregate DTA-ABM integration could then be considered at a later stage, which would require a significant amount of restructuring to both ABM (SF-CHAMP) and the DTA model. The panel felt that a regional level DTA model might involve a huge amount of work and may even be unnecessary

The panel feels the SFCTA may not choose to implement some of the long term recommendations because they may be beyond the scope of the current DTA project.

Updated: 1/23/2014
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