Introduction
The Transportation Authority uses a Dynamic Traffic Assignment (DTA) model to give planners a more fine-grained view of transportation system performance and a better understanding of traffic patterns around San Francisco. DTA is an intermediate-scale simulation of both autos and transit vehicles that the Transportation Authority has previously piloted for a few projects in the northwestern section of the city. In 2012, the Transportation Authority completed the development of a citywide DTA model with the help of a grant from the Federal Highway Administration.
Using DTA for Transportation Modeling in San Francisco
The Transportation Authority needs a tool that can effectively predict changes in travel across our network in order to evaluate the effects of various transportation projects. While the Transportation Authority's SF-CHAMP travel demand forecasting model has a very detailed understanding of the intricate travel demand decisions of individuals for our entire region, it is limited in its ability to capture the detailed behavior of traffic and buses as they respond to traffic signal timing, queue formation, and other detailed operational information. Just as more sophisticated activity-based travel demand models (such as SF-CHAMP) have better sensitivity to non-standard transportation improvements than traditional four-step models, DTA provides the sensitivity and robustness needed to analyze traffic operational strategies and their outcomes, such as travel time reliability.
Examples where the Transportation Authority has already used DTA are:
- Determining where cars would reroute during long-term road closures for construction.
- Evaluating bus travel times and auto route shifts for a Bus Rapid Transit project.
The DTA Anyway Project
The DTA Anyway project, funded by a grant from the Federal Highway Administration, was completed in November 2012.
The project simultaneously:
Created an open-source code base to lower the barrier to entry to large-scale DTA modeling and reduce the amount of effort necessary for maintaining it.
Researched how a large-scale DTA model works, both by itself and when integrated with an activity-based travel demand model.
Developed a working and calibrated DTA model of San Francisco County for the numerous projects that would benefit from it.
The DTA Anyway team shared the ins and outs of the project at the DTA Anyway project website, which was revised several times a week with new code and project updates.
DTA Anyway was a huge undertaking, requiring a massive amount of data synthesis and maintenance as well as model calibration to make sure the model is behaving reasonably. While there are a few large-scale DTA networks in existence, most are built to evaluate a single project or as a research demonstration. Ours is likely the first DTA model of this scale built to be continuously maintained and used by a public agency, and by doing it "out in the open" on our project website, we hope to lower the barrier to entry for other agencies.
Resources
Transportation Authority Reports
DTA Final Report, 2012 (PDF)
DTA Calibration and Validation Report, 2012 (PDF)
DTA Future Research Topics (PDF)
Publications and Presentations
Dynamic Traffic Assignment Primer, June 2011
Feasibility of using Citywide DTA in San Francisco, 2009 (PDF)
DTA Model Development Webinar Presentation, 2012 (PDF)