Dynamic Traffic Assignment (DTA)

AN INNOVATIVE TRAFFIC MODEL FOR A DISTINCTIVE CITY

The Transportation Authority has recently been using a technology called Dynamic Traffic Assignment (DTA) to give planners a more fine-grained view of transportation system performance as well as a better understanding of the how-and-why of traffic routing itself 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 Fall 2012, the Transportation Authority completed the development of a citywide DTA model with the help of a grant from the Federal Highway Administration (FHWA).

WHAT IS DTA AND WHY DO WE WANT IT IN SAN FRANCISCO?

The Transportation Authority needs a tool that can effectively predict changes in travel across our grid 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. DTA models incorporate transportation system performance details such as traffic signal timing, queue formation, and route choice decisions—important considerations when analyzing projects in San Francisco. 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 a number of other important items: congestion, diversion analysis (e.g., where does traffic on street X go when you implement project Z?), and travel time reliability.

DTA provides a strong behavioral link between the regional and citywide investments and policies captured in SF-CHAMP and the local design and engineering measures typically evaluated at an intersection or at the block-by-block level. It allows us to more precisely and robustly measure the local impacts of large, regional projects as well as the regional impacts of local projects. While SF-CHAMP knows how many lanes are on a street and what the speed limit is, DTA understands additional details such as how the traffic signals are timed, what side of an intersection the bus stops on, and how oncoming traffic affects left-turning vehicles.

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.

PRESENTATIONS

REPORTS

RELEVANT PUBLICATIONS

WHAT IS DTA ANYWAY?

The DTA Anyway project, funded by a grant from FHWA, was completed in November 2012. The project simultaniously:

  • 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.

FOR MORE INFORMATION

Contact Elizabeth Sall, Deputy Director for Technology Services.