Ramp flow control (also known as ramp metering) uses red lights on entrance ramps to let vehicles onto a freeway in a smooth, even manner. Vehicles wait on the ramp until the light changes. By releasing one or two vehicles at a time, ramp flow control prevents entering traffic from crowding out freeway traffic, stop-and-go traffic, and its ripple effects. This strategy may not stop all congestion, but it can delay its onset and shorten it.