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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 traffic entering freeways according to current traffic conditions. They are intended to reduce congestion on the freeway in two ways. One is to break up platoons of vehicles entering freeways, ensuring that traffic can merge more easily. A second is to ensure that total flow on the freeway does not exceed capacity at downstream bottlenecks. Some metered ramps have bypass lanes for high occupancy vehicles, allowing carpoolers and vehicles like 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 parallel lane of traffic entering the freeway; 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 enterance 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). Ramp metering in North AmericaIn 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, Minneapolis-St. Paul, Minnesota, Seattle, Washington, Denver, Colorado, Phoenix, Arizona, Salt Lake City, Utah, Portland, Oregon, 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. 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. Ramp metering in AustraliaRamp meetering is used to regulate access to a number of major roads in Sydney, including:
Ramp metering is dynamic and only activates under certain traffic conditions. External linksWhat does Ramp meter mean ? Search with Google !Article on Ramp meter, category, different spelling or sense |
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