About Ramp Metering
What is Ramp Metering?
Ramp meters are traffic signals installed on freeway on-ramps to control the frequency at which vehicles enter the flow of traffic on the freeway. As seen in the diagram below, vehicles traveling from an adjacent arterial onto the ramp form a queue behind the stop line. The vehicles are then individually released onto the mainline, often at a rate that is dependent on the mainline traffic volume and speed at that time. The configuration in the diagram is the most common; however, some agencies have altered this design to accommodate transit and high-occupancy vehicle (HOV) policies or existing geometric limitations.
Source: Parsons Brinckerhoff.
How They Work
Without ramp meters in operation, multiple vehicles merge in tightly packed platoons, causing drivers on the mainline to slow down or even stop in order to allow vehicles to enter. The cascading slower speeds, both on the mainline and on the ramp, quickly lead to congestion and sometimes stop-and-go conditions. Ramp meters can break up the platoons by controlling the rate at which vehicles enter the mainline from the ramp, as shown in the figure to the right. This allows vehicles to merge smoothly onto the mainline and reduces the need for vehicles on the mainline to reduce speed.
In addition to breaking up platoons, ramp meters help manage entrance demand at a level that is near the capacity of the freeway, which prevents traffic flow breakdowns. Ramp meters are shown to reduce peak hour occupancies and quicken recovery from mainline breakdown back to or below the critical occupancy threshold. Typical results include reductions in travel time, reductions in crash rates, and increased traffic speed.
Source: Washington State Department of Transportation.
United States Department of Transportation - Federal Highway Administration