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Playing in Traffic

Can warning drivers of traffic jams make congestion worse? Can closing roads make it better? Mathematically yes, but real-world confirmation is hard to find.

Brian Hayes

I was southbound on Interstate 95, approaching Washington, DC, on a summer afternoon. The Capital Beltway offers two routes around the city—an eastern loop via Greenbelt, MD, and a western arc through Tysons Corner, VA. As I approached the decision point, online traffic maps showed several slow spots on the western branch, whereas the eastern roadway was flowing freely. The choice seemed clear, yet an unsettling thought kept nagging at me: Other drivers had access to the same information I was seeing. If we all followed the recommended route, our strategy would be self-defeating. In collectively avoiding one traffic jam, we’d create a new one.

Traffic patterns present many such puzzles and perplexities. (Pondering them can help pass the time when you’re caught in gridlock.) One of the most intriguing ideas in the theory of transport networks is Braess’s paradox, which says that building a new road to relieve congestion can sometimes have the opposite effect, causing greater delays for all drivers. Conversely, closing off a road can sometimes speed everyone’s journey.

In trying to better understand these counterintuitive cloggings and clearings of roadways, I have been playing with computer simulations in which I can watch individual vehicles wend their way through a network of roads, choosing a path at each intersection. Although the model is a simple one, it does show evidence of Braess’s paradox, along with an abundance of other curious instabilities and oscillations. Whether the results will help drivers—or future driverless vehicles—navigate real highways remains to be seen.

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