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Center for Transportation Studies

Relieving Congestion and Saving Energy by Cooperative Intelligent Transportation Systems

Presentation by Steven Shladover, Research Engineer, California Partners for Advanced Transit and Highways (PATH), University of California, Berkeley

October 8, 2009

As our roads fill with smart cars zipping past freeway signs with real-time traffic data, researchers are looking at the next step—using direct communication among vehicles and between vehicles and the roadside. The vehicle-infrastructure integration (VII) initiative— now known as IntelliDrive—was set up to alert drivers about roadway hazards. But this cooperative ITS technology can also be used to relieve congestion and save energy, which are the goals of the California Partners for Advanced Transit and Highways (PATH) Program project titled “Development and Evaluation of Mobility Applications for VII.” The research project is sponsored by the FHWA Exploratory Advanced Research Program and Caltrans.

In the PATH VII project, researchers are working on three cooperative technology applications: active traffic management, cooperative adaptive cruise control, and automated truck platooning. On October 8, PATH research engineer Steven Shladover presented current work on these applications as part of the ITS Institute's Advanced Transportation Technologies seminar series.

The key to a VII system is information flow among vehicles, the roadway, and the people in the vehicles, Shladover said. When vehicles communicate motions and intentions to each other, it allows maneuvers such as coordinated merging and car following, which can improve traffic flow. When vehicles communicate with the roadway infrastructure, it offers better real-time data than roadway sensors alone can provide. In addition, traffic managers can communicate with drivers and vehicles to signal crashes and slowdowns or to control traffic flow and prevent bottlenecks.

The first stage of the PATH project focused on active traffic management, using ramp metering and variable speed limits to control the speed and density of traffic to avoid a traffic breakdown, Shladover said.

The project focused on an eight-mile section of Interstate 80 approaching the San Francisco–Oakland Bay Bridge, combining traffic models with real-time data provided by the Berkeley Highway Lab. The team tried to push the highway system models as close as possible to capacity in bottleneck areas while avoiding a breakdown.

Simulation results showed that the variable speed limits allowed drivers to maintain speeds, even during peak periods, and helped control traffic density. In models, total travel time was reduced by 32 percent and average flow rate was increased by 13 percent over a 10-hour period including both peak and off-peak conditions.

“This is looking promising in that by choosing the speed limit appropriately we can avoid getting into the flow breakdown,” Shladover said.

Stage two of the project involved using human drivers to test whether adaptive cruise control (ACC) and cooperative adaptive cruise control (CACC) could close gaps between cars, leading to higher lane capacity, fewer cut-ins by other vehicles, and improved traffic flow.

Previous traffic simulations had shown that cars using CACC with a half-second time gap could double lane capacity. A key question of the PATH study was whether drivers would accept a half-second gap or would prefer a longer distance between cars.

In a two-week study, 16 drivers used cars equipped with ACC and CACC on their daily commutes. Drivers were able to choose the time gap they preferred with other vehicles and could deactivate the systems whenever they preferred.

Measured data and follow-up surveys showed that drivers in the CACC cars were generally most comfortable choosing 0.6- or 0.7-second gaps between cars. The results showed that drivers rated ACC and CACC similarly and preferred both over conventional cruise control or manual driving. “The people who had a chance to drive it really love it,” Shladover said.

A third component of the research focused on truck platooning. Prior PATH research showed that platoons of two tractor-trailers realized energy savings of 10 to 15 percent and could double the capacity of a truck-only lane.

The current work extends the research to three trucks and uses dedicated short-range communication instead of Wi-Fi for vehicle-to-vehicle communication. Based on wind-tunnel experiments and past tests, PATH researchers expect the three-truck platoon to create even more energy savings, Shladover said. The research will be completed within a year.

Next steps for the PATH project include applying the active traffic management approach to a wider variety of road conditions, conducting more field tests with drivers in CACC cars, and designing ways to ensure that failures in truck platoons do not create safety problems, Shladover said.