Ramp meter

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. 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.[2]

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.

Ramp meter signals are set according to the current traffic conditions on the road. Detectors (generally an induction loop) are installed in the road, both on the ramp and on the main road which measure and calculate the traffic flow, speed and occupancy levels. These are then used to alter the number of vehicles that can leave the ramp. The more congested the main freeway, the fewer vehicles are allowed to leave the ramp, this is effected by giving longer red times to the traffic signals.

Much research is currently being carried out into the most appropriate algorithms for controlling ramp meter signals. Some algorithms that are in use or have been evaluated are ALINEA, demand control and fuzzy algorithms.

This first application involved a police officer who would stop traffic on an entrance ramp and release vehicles one at a time at a predetermined rate, so that the objectives of safer and smoother merging onto the freeway traffic was easier without disrupting the mainline flows.

Ramp metering was first implemented in 1963 on the Eisenhower Expressway (Interstate 290) in Chicago by Adolf D. May, now a UC Berkeley professor.[3] Development in systems control theory allowed for improved traffic regulation throughout the early 1970s, pioneered by Leif Isaksen in his paper "Suboptimal Control of Large Scale Systems with Application to Freeway Traffic Regulation."[4] Since then ramp-meters have been systematically deployed in many urban areas including Los Angeles; San Diego; Sacramento; the San Francisco Bay Area; Fresno; Philadelphia, Pennsylvania; Seattle; Spokane;[5] Denver; Phoenix; Las Vegas; Salt Lake City; Portland, Oregon; Minneapolis-St. Paul; Milwaukee; Columbus; Cincinnati;[6] Houston; Atlanta; Miami; Washington, DC (only along Interstate 270 in Montgomery County, Maryland and Interstate 395 and Interstate 66 in Arlington County, Virginia); Kansas City, Missouri;[7] and along the Queen Elizabeth Way in Mississauga, Ontario (Toronto-bound ramps from Cawthra Road, Hurontario Street, Mississauga Road, Erin Mills Parkway, Winston Churchill Boulevard, Ford Drive) Canada since the 1970s.[2] In the early 1970s, this traffic control practice drew the attention of the U.S. Environmental Protection Agency, which was looking for innovative ways to reduce air pollution in California by make the transportation system more effective.[8]

A Milwaukee, Wisconsin ramp meter

Ramp meters are commonplace in the New York City, Los Angeles, San Francisco, Chicago, Seattle, Phoenix, Houston, Atlanta, Milwaukee, Columbus,[9] and Minneapolis-St. Paul metropolitan areas, and they are also found in more than two dozen smaller metropolitan areas. In the New York City metro area, locals refer to ramp meters as "merge lights" and in Houston they're known as "flow signals."

Ramp meters have been withdrawn after initial introduction in several cities, including Dallas, San Antonio, and Austin, Texas. Disused metering signals can still be found along some parkways surrounding New York City and Detroit. Although deactivated shortly after they were added, ramp meters have been reactivated at select interchanges of Interstate 476 in suburban Philadelphia.

Ramp meters were installed along Interstate 435 in Overland Park, Kansas and Kansas City, Missouri in 2009.[7]

Ramp meters in Mississauga, Ontario are designed in such a way so that if the queue waiting to enter the QEW grows to the point where it may back up onto city streets, the meter is lifted and all traffic entering the highway is able to move freely without waiting for the meter. The meter goes back into service once the ramp queue is reduced to a reasonable level. While this method may increase congestion on the highway itself, it has the benefit of keeping city arterials free of stopped traffic waiting in queue. Ramp queues are usually quite short, lasting only 5–6 seconds on average before the driver may continue to the freeway.

Ramp metering has been installed in several countries in Europe, including the United Kingdom, Germany and the Netherlands. A research project, EURAMP - European Ramp Metering Project, funded by the European Union was completed in March 2007. The EURAMP Project Deliverables[10] included information about the results of ramp metering in a number of locations and situations, and whether they were helpful in those situations, and a Handbook of ramp metering.

United Kingdom

Ramp Metering on the A19 in Middlesbrough.

The first trial in the UK was on the M6 J10 near Walsall in 1986. No more sites were developed for the next two decades until a second 'pilot' study in 2006 by the Highways Agency (HA) concluded that ramp metering provides a net benefit under certain conditions - generally more congested junctions. A Summary Report[11] by the HA, dated November 2007, includes an overview of the background and history, international experience, limitations, system operation, algorithms and implementation of ramp metering. In its conclusion it "envisaged ramp metering will be deployed more widely in the coming period." Ramp metering was then introduced widely in England - Phase 1 involved the implementation of approximately 30 sites and was completed by 2008. Phase 2 followed and as of March 2011 there are 88 Ramp Metering sites[12] on the 4,500 miles (7,242 km) of strategic highways operated and maintained by the HA.

On some ramp meters, there is a singular red light on the backside of the signal that is synchronized with the red light on the traffic signal that the drivers see when queueing. When the queueing drivers see the red light on the traffic signal, the backside red light is on, when the traffic signal displays green or yellow, the backside red light is off. This allows highway patrol to enforce the metering lights by having an officer park their car or motorcycle on the shoulder a short distance past the targeted signal, watch the red light for any offenders, and pull them over. Additionally, if the given ramp has a carpool bypass lane, officers from that vantage point may also catch and pull over non-carpoolers who are illegally using the carpool lane to skip past the ramp meter queue. On westbound I-80 in Oakland, California just past the Bay Bridge toll plaza, there is a section with overhead metering lights that cycle when the freeway traffic volumes are high, equipped with cameras that capture license plate images of drivers who run the red light and then send a fine.

• University of Minnesota
• Ramp Signalling - Auckland Ramp Signalling information
• Ramp Meters in the U.S.
• VicRoads Managed Freeways - Freeway Ramp Signals Handbook