Ramp meter
A ramp meter, ramp signal, or metering light is a device, usually a basic traffic light or a two-section signal light (red and green only, no yellow) together with a signal controller, that regulates the flow of traffic entering freeways according to current traffic conditions. Ramp meters are used at freeway on-ramps to manage the rate of automobiles entering the freeway. Ramp metering systems have proved to be successful in decreasing traffic congestion and improving driver safety. 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. Some ramp meters are designed and programmed to operate only at times of peak travel demand; during off-peak times, such meters are either showing a steady green, flashing yellow (Maryland), or are turned off altogether. This allows traffic to merge onto the freeway without stopping. Other ramp meters are designed to operate continuously, only being turned off for maintenance or repairs. Some metered ramps have bypass lanes for high-occupancy vehicles, allowing carpoolers, buses, and other eligible vehicles to skip the queue and get directly on the highway. In other places such as Northern California, carpool lanes are still metered, but the queue is typically shorter in comparison to regular lanes. 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 vehicle at the front of the queue.