Why does traffic bottleneck on freeways for no apparent reason?
When something disturbs the normal course of traffic, the effects can last for a surprisingly long time after the incident itself is gone, and affect areas far from the initial problem. “Maybe a dog runs into the road,” says Moshe Ben-Akiva, professor of Civil and Environmental Engineering and director of the MIT Intelligent Transportation Systems program. “Maybe people slow down to look at something. Maybe someone cuts someone off and they start arguing. The cars at the front get moving again after a couple of minutes, but cars behind them still have to stop and queue up. It’s like a shock wave that moves upstream.” The phenomenon is portrayed eloquently in MITSIMLab, a traffic simulator developed at MIT in the 1990s with funding from Boston’s Big Dig highway project. During a recent demonstration of the software, graduate student Samiul Hasan set up an animated map of downtown Boston highways to simulate traffic on a typical weekday morning. Thousands of colored rectangles representi
