Unsourced material may be challenged and removed. It is the use of traffic signals at freeway on-ramps to manage highway engineering handbook pdf rate of automobiles entering the freeway.
Other ramp meters are designed to operate continuously, only being turned off for maintenance or repairs. In other places such as Northern California, carpool lanes are still metered, but the queue is typically shorter in comparison to regular lanes. Generally, meters with multiple lanes only give one lane the green light at a time. 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. In California, some meters allow two or three cars to proceed on a green light. These meters use red-yellow-green signals on both the upper and lower mounts on the pole, and operate in a standard green-yellow-red fashion. In Ontario, the ramp meter lights are always green when there are no restrictions in place for traffic to proceed. The sophistication and extent of a ramp metering system should be 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 effectively. 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 that enter the freeway. Presence and passage detectors may be installed on the ramp to actuate and terminate the metering cycles, but the metering rate is based on average traffic conditions at a particular ramp at a particular time. This type of operation provides the benefits associated with accident reductions, but is not as effective in regulating freeway volumes because there is no input about mainline traffic. 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. The next level of control, traffic responsive, establishes metering rates based on actual freeway conditions.
The local traffic responsive approach utilizes detectors and a micro-processor to determine the mainline flow in the immediate vicinity of the ramp and the ramp demand to select an appropriate metering rate. Traffic responsive control also permits ramp metering to be used to help manage demand when incidents occur on the freeway, i. System-wide control is a form of traffic responsive control but operates on the basis of total freeway conditions. Centralized computer controlled systems can handle numerous ramps in a traffic responsive scheme and feature multiple control programs and overrides.