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Rumble strips are grooves in pavement, typically constructed on the shoulder of a highway perpendicular to the direction of travel. When an automobile's tires roll over them, a loud, obnoxious noise is produced and the vehicle shakes rapidly. The point is safety: drivers who doze off and begin to drift off the road will be shocked into attention and alertness.

Typically several inches deep and spaced in short intervals, rumble strips do decrease auto rollover accidents. So they're good for drivers. Bad for bicyclists, unfortunately, since a bike that rolls over them can easily lose control and toss its rider into traffic. It seems to be an issue of cyclists versus traffic engineers here - bicycle organizations want rumble strips removed from roads where bike travel is allowed, while highway planners (who really only care about the people using motorized transportation) want to put them everywhere they can.

Only the most sadistic road builders put rumble strips along the main lanes of travel. In such cases, they're used as an alternate to speed bumps, forcing vehicles to slow down (under penalty of extreme irritation) while approaching toll booths or raised bridges.