speed bump

n.

A speed bump (British English a speed or road hump, sometimes colloquially a sleeping policeman) is a traffic calming tool designed to slow traffic or reduce through traffic. A speed bump is a bump in a roadway that may be circular, parabolic, or sinusoidal, and it may have gaps near the curb to allow drainage. The speed at which a vehicle can safety pass a bump decreases with the bump's slope.