Ramp meter

A ramp meter is a device that regulates traffic entering freeways according to current traffic conditions. They have several aims. One is to break up platoons entering freeways, ensuring that traffic can merge easily. A second is to ensure that total flow on the freeway does not exceed capacity at downstream bottlenecks. In the Los Angeles 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, 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 Ramp meters have been withdrawn after initial introduction in several cities, including Austin, Texas, Dallas, Texas and San Antonio, Texas and Columbus, Ohio.

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.