Machine Design

Algorithm solves problems of phantom traffic jams

Everyone has experienced traffic backups on the freeway, where traffic almost comes to a stop, and miraculously thins out with no sign of an accident, construction, or a police car with its radar out. A computer scientist at the Massachusetts Institute of Technology has figured out a cure for such “traffic flow instabilities,” a common problem that has been recognized and studied since the 1930s. All it takes is “bilateral control,“ a form of adaptive cruise control smart enough to keep a car halfway between the vehicles in front and behind.

These phantom traffic jams arise because speed variations get magnified as they pass down a lane of traffic, says Betrhold Horn, an MIT professor and author of the idea. If a driver hits the brakes hard for just a moment, that action gets propagated upstream and increases in amplitude. It acts like there is positive feedback and small perturbations can get it started.

Mathematical modeling of traffic and the bilateral-control algorithm proved the idea would work. Unfortunately, there are several roadblocks to using it in the real world. First, almost every driver must have and use bilateral cruise control on the highways. Second, no vehicles currently have bilateral control. Some upscale cars have adaptive cruise control that keeps them a set distance from the car in front, but these systems currently ignore traffic behind them. Adding the capability would mean additional sensing hardware and computational power.



For an animation showing the algorithm in action:

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