Pravin Varaiya

About Pravin Varaiya (Edit profile)

Pravin Varaiya is Professor of Electrical Engineering and Computer Science at the University of California, Berkeley (varaiya@eecs.berkeley.edu).

What We’ve Learned About Highway Congestion

Pravin Varaiya

There are 26,000 SENSORS buried under the pavements of California freeways. Every thirty seconds, those sensors send data to our computers here in Berkeley. The data tell us about the number of cars driving on that freeway and their speed at that time. We also collect, process, and store data about collisions and other incidents. This database, PeMS (Performance Monitoring System), is now by far the most comprehensive source of information about California highways. Today it stores four trillion bytes of information, which are available online at http://pems.eecs.berkeley.edu. We’ve already learned quite a lot from all those data. For example, we’ve found the error in the old belief that an average speed of 40 to 45 mph maximizes traffic capacity; we now know for a fact that maximum capacity occurs at around 60 mph. And we’ve been surprised to discover that some HOV lanes may have the perverse effect of actually adding to congestion.

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THE ACCESS ALMANAC: The Freeway-Congestion Paradox

Chao Chen and Pravin Varaiya

Traffic congestion on a freeway sets in once the density of vehicles exceeds a certain critical number. Above that, both vehicle speed and vehicle flow drop precipitously. Well-designed ramp meters can limit the number of vehicles entering a freeway, so that critical density is not reached, congestion is avoided and, paradoxically, both speed and flow increase. This double gain of reduced travel time and increased flow far exceeds any improvements that can be achieved by constructing more freeway lanes.

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An Eye on the Fast Lane: Making Freeways Work

Pravin Varaiya

Before leaving for work you can check the weather from the newspaper, radio, or TV report. You can just look at the sky and make your own guess. But you can’t determine much about current traffic conditions. TV and radio traffic reports provide only a summary (“traffic is running smoothly this morning”) and spotty coverage of incidents (“an accident in the second lane has been cleared”). If you are unexpectedly stalled in traffic, your frustration and anxiety mount. You don’t know how long you’ll be stuck or whether you should use your cell phone to cancel your appointment.

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