California Department of Transportation


Q Why are there signals installed at some freeway onramps?

Stopping cars momentarily at on-ramps can save motorists several minutes per trip because overall freeway congestion is reduced. "Ramp meters" are a familiar sight to California motorists, having made their debut in 1966 at the intersection of Routes 5 and 14 in northern Los Angeles County. Since then, the ubiquitous signals at urban on-ramps have helped regulate the smooth flow of vehicles on to the freeway and thereby ensure that freeways operate at maximum capacity. Today, more than 1,000 ramp meters are in operation statewide. Recently, the concept of metering has been expanded to help alleviate congestion at freeway interchanges, which have traditionally experienced some of the worst congestion on the freeway system. Freeway-to-freeway connector ramp meters were first put in use in 1978 along the westbound Route 94 corridor in San Diego. In 1992, a freeway-to-freeway connector ramp meter was installed in Los Angeles County at the transition from southbound Route 5 and westbound Route 110. The Glenn Anderson (105) Freeway is the first freeway to employ connector ramp metering on full interchanges when it opened in 1993. These signals in some cases employ the traditional red-amber-green signalization typical of local street intersections, rather than the red-green lights more common on on-ramp meters. Connector ramp meters were put in place at four interchanges along Interstate 105: At the San Diego (405) Freeway, the Harbor (110) Freeway, the Long Beach (710) Freeway and the San Gabriel River (605) Freeway. Flashing overhead warning signs alert motorists that the meters are ahead as they make the transition from one freeway to another. Sensors embedded in the pavement are linked to computers, which will adjust the meters automatically to optimize the movement of traffic through the interchange. Motorists come to appreciate the value of ramp meters when they don't work -- on the few occasions when they have been shut off due to malfunction or maintenance, allowing cars to enter the freeway at random, freeway traffic can backup for miles.