Freeway Management Program
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Freeway Management Program > Ramp Management and Metering > Ramp Metering

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.

A graphic showing a ramp metering configuration. The overcrossing arterial has a ramp that enters into the right lane of a three-lane mainline. There is a ramp meter and stop bar located near where the ramp enters onto the mainline. There is an advanced queue sensor loop located before the ramp entrance in the arterial; a loop detector on the ramp; another demand loop at a cars length from the ramp meter and stop bar; a passage loop located after the stop bar at the ramp entrance to the mainline; and sensor loops in each lane of the mainline. All loops lead to a controller cabinet which is positioned between the ramp and the mainline.

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.

A graphic showing a comparison of mainline conditions with and without ramp metering: both a three-lane free way with an entrance ramp on the right without ramp metering, and a three-lane freeway with an entrance ramp on the right with ramp metering are shown. The freeway without ramp metering shows significant congestion in the right lane prior to where the entrance ramp joins, as well as some congestion in the other lanes before the ramp joins. The freeway with ramp metering shows an even traffic flow in all lanes, both before and after the entrance ramp joins.

Source: Washington State Department of Transportation.
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