Why do Freeways Come to a Stop?

Why do freeways come to a stop? EXIT Cleveland THIS LANE ONLY Traffic is rolling along at 60 mph when someone slows to 50 mph. In this example, the driver of Car B does so to avoid hitting Car A, whose driver swerves at It happens to most drivers at least a few times a year. You're sailing along on the freeway when you're forced to come to a stop, or at least a crawl. You can't see why things are slowing around the bend – and when you get there, traffic is moving better. the last second to exit. Traffic planners call this a "shockwave." The next driver slows to 45 mph to maintain a safe distance from cars A and B. э Drivers farther back see the brake lights and begin slowing down. The funnel effect Cuyahoga County transportation engineer Jamal Husani compares rush-hour traffic to a funnel. Just the right amount of water can go through as fast as it's put in the funnel. The pattern continues, and more drivers apply their brakes until traffic comes to a crawl. By the time the rear of But add extra water to the funnel, and the whole thing backs up. the jam catches up to where the shockwave began, the offending parties are long gone and there is no sign of what caused the problem. "The first few drivers could have a big impact," Husani said. "Their behavior in the peak time has a huge ripple effect, even if it doesn't look that bad to them." STEPHEN J. BEARD AND RICH EXNER | THE PLAIN DEALER