Ramp meter

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. Some metered ramps have bypass lanes for high-occupancy vehicles, allowing carpools, buses, and other eligible vehicles to skip the queue and get directly on the highway. The sophistication and extent of a ramp metering system is based on the amount of improvement desired, existing traffic conditions, installation costs, and the continuing resource requirements that are necessary to operate and maintain the system. The simplest form of control is a fixed time operation. It performs the basic functions of breaking up platoons into single-vehicle entries and setting an upper limit on the flow rates. The next level of control, traffic responsive, establishes metering rates based on actual freeway conditions. This type of control provides the benefits associated with accident reductions, but is not as effective in regulating freeway volumes. Pre-timed control can be implemented on any number of ramps and is often implemented as an initial operating strategy until individual ramps can be incorporated into a traffic responsive system.