Ramp Meter - Types

Types

Some metered ramps have bypass lanes for high-occupancy vehicles, allowing carpoolers and buses to skip the queue and get directly on the highway. 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. 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.

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.e. reduce the metering rate at ramps upstream of the incident and increase the rate at ramps downstream. 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. Control strategies can also be distributed among individual ramps. A significant feature of system control is interconnection that permits the metering rate at any ramp to be influenced by conditions at other locations. Denver showed that this type of control has significant benefits when properly applied.’

System control need not be limited to the freeway and its ramps. The concept of integrated traffic control combines or coordinates freeway and arterial street control systems to operate on the basis of corridor wide traffic conditions. The potential advantages of integrated control include reduced installation and operating costs, corridor wide surveillance, better motorist information, and quicker and coordinated use of all of the control elements (meters, signals, signs, etc.) in response to real time traffic conditions. Simulation results from one study showed that, during an incident, coordination of arterial traffic signals and ramp meters can improve the traffic performance of a corridor.