Ramp meter

From Free net encyclopedia

A ramp meter or metering light is a device, usually a basic traffic light or a two-phase (red and green, no yellow) light, that regulates the flow of traffic entering freeways according to current traffic conditions. They are intended to reduce congestion on the freeway in two ways. One is to ensure that the total flow entering the freeway does not exceed the capacity at a downstream bottleneck. A second is to break up platoons of vehicles entering freeways, ensuring that traffic can merge more easily. Some metered ramps have bypass lanes for high-occupancy vehicles, allowing carpoolers and buses to skip the queue and get directly on the highway. Meters often only operate in rush hour periods.

Some ramp meters have only one lane of traffic at the signal; others may have two or more lanes of traffic. Generally, meters with multiple lanes only give one lane the green light at a time. In one common configuration, each entrance lane has two signals; a red-yellow-green signal perched overhead over each lane (or mounted high on a pole for a single lane), and a two-phase lamp mounted low on a pole next to the stop line. The overhead lights are for cars approaching the metering point; the low-mounted two-phase lights are intended to be used by the vehicles at the front of the line. In normal operation of the ramp meters, only the red and green lamps are used. However, when ramp metering is about to be enabled, the overhead lamps may show flashing or solid yellow to warn drivers to prepare to stop. (Once ramp metering is turned on, there is no further need for the yellow lamp).

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Ramp metering in North America

In the Los Angeles, Phoenix, and Minneapolis-St. Paul metropolitan areas they are commonplace, and they are found in more than two dozen smaller metropolitan areas.

Ramp metering was first implemented in 1963 on the Eisenhower Expressway (Interstate 290) in Chicago, Illinois. This first application involved a police officer who would stop traffic on an entrance ramp and release vehicles one at a time at a predetermined rate, so that the objectives of safer and smoother merging onto the freeway traffic was easier without disrupting the mainline flows. Since then ramp-meters have been systematically deployed in many urban areas including Los Angeles, California, San Diego, California, the San Francisco Bay Area, Minneapolis-St. Paul, Minnesota, Seattle, Washington, Denver, Colorado, Madison, Wisconsin, Atlanta, Georgia, Phoenix, Arizona, Salt Lake City, Utah, Portland, Oregon, Washington, DC (only along Interstate 395), Interstate 66 in Arlington County, Virginia, and Toronto, Ontario.

Ramp meters have been withdrawn after initial introduction in several cities, including Austin, Texas, Dallas, Texas, and San Antonio, Texas and Columbus, Ohio. Disused metering signals can still be found, forgotten, along some parkways surrounding New York, New York and Detroit, Michigan. Although deactivated shortly after they were added, ramp meters have been reactivated at select interchanges of Interstate 476 in Philadelphia, Pennsylvania.

In 2000, an experiment was conducted involving shutting off all 433 ramp meters in the Minneapolis-St. Paul area for eight weeks to test their effects. In general, ramp meters were shown to reduce accidents and marginally reduce total travel time compared with the unmetered case. However, they remained controversial, and the Minnesota state Department of Transportation has developed new, less onerous ramp control strategies. Fewer meters are activated during the course of a normal day than what had happened prior to 2000, and timing has been altered.

A mainline meter, which is a specialized form of metering, has been implemented at the San Francisco-Oakland Bay Bridge toll plaza.

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