Ramp meter

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A ramp meter, ramp signal or metering light is a device, usually a basic traffic light or a two-section signal (red and green only, no yellow) light together with a signal controller, that regulates the flow of traffic entering freeways according to current traffic conditions.

Ramp meters are claimed to reduce congestion (increase speed and volume) on freeways by reducing demand and by breaking up platoons of cars. Two variations of demand reduction are commonly cited; one being access rate, the other diversion.[1]



Ramp meters are installed to restrict the total flow entering the freeway, temporarily storing it on the ramps, a process called "access rate reduction." In this way, the traffic flow does not exceed the freeway's capacity. Another rationale for installing ramp meters is the argument that they prevent congestion and break up "platoons" of cars. A platoon is a group of vehicles traveling in proximity, such as a group released by an arterial traffic signal changing from red to green. Advocates of ramp meters claim that they break up platoons of vehicles entering freeways, ensuring that traffic can merge more easily. Another premise of ramp meters is diversion. The delay caused by the ramp meter waiting period may cause some drivers to choose other routes thereby reducing demand for the freeway.[2]

Criticisms of ramp metering primarily center on impacts to users nearer to the urban core, which are typically delayed by the ramp metering, as compared to often uncontrolled users approaching from further out along the system. Such considerations may raise questions of equity, as was prevalent with the Milwaukee system. In the longer-term, it is conceivable that population growth may be encouraged further from the urban core and discouraged nearer, potentially encouraging sprawl development and increasing fuel usage. Additional concerns include queued traffic overflowing out of the ramps and onto other streets approaching the interchange. However some cities, like Atlanta, have fixed this by having the meters shut off temporarily to "flush out" the ramp once the traffic hits the streets. The meters then turn back on once the ramp has been cleared. Another concern is the increased pollutant emissions arising from the metered traffic[3] and from the additional emissions from accelerating from a stop to freeway speeds (versus accelerating from street speeds to freeway speeds).

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