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 groups 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.
Types 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. 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.
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 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 metering signal controls 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.
Demand control algorithms The demand control algorithms are examples of feed-forward control. One version of the demand control algorithm is the RWS strategy used in the Netherlands. In this algorithm the number of vehicles that the signals allow off the ramp is calculated as the difference between the flow before the ramp and the pre-specified capacity of the road.
Ramp metering in North America 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.
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." 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; Denver; Phoenix; Las Vegas; Salt Lake City; Portland, Oregon; Minneapolis-St. Paul; Milwaukee; Indianapolis; Columbus; Cincinnati; Houston; Atlanta; Miami; Orlando; Washington, DC (only along Interstate 270 in Montgomery County, Maryland and Interstate 395 and Interstate 66 in Arlington County, Virginia); Kansas City, Missouri; 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. 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.“Early Implementation of the Clean Air Act of 1970 in California.” EPA Alumni Association. Video, Transcript (see p11). July 12, 2016.
Ramp meters are commonplace in the New York City, Los Angeles, San Francisco, Chicago, Seattle, Phoenix, Houston, Atlanta, Milwaukee, Columbus,Ramp Meters 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. In 2017, ramp meters were installed along Interstate 35 in Kansas City.
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.
Minneapolis–Saint Paul ramp meter experiment
In 2000, a $650,000 experiment was mandated by the Minnesota State Legislature in response to citizen complaints and the efforts of State Senator Dick Day. The study involved shutting off all 433 ramp meters in the Minneapolis-St. Paul area for eight weeks to test their effectiveness. The study was conducted by Cambridge Systematics and concluded that when the ramp meters were turned off freeway capacity decreased by 9%, travel times increased by 22%, freeway speeds dropped by 7% and crashes increased by 26%. However, ramp meters remain controversial, and the Minnesota State Department of Transportation has developed new ramp control strategies. Fewer meters are activated during the course of a normal day than prior to the 2000 study, some meters have been removed, timing has been altered so that no driver waits more than four minutes in ramp queue, and vehicles are not allowed to back up onto city streets.
Mainline metering A mainline meter throttles traffic flow from one segment of a highway to the next by directly metering the highway's traffic. Such a scheme is typically implemented in specialized situations such as bridges and tunnels. A mainline meter was installed at the San Francisco–Oakland Bay Bridge toll plaza in the early 1970s. Similar mainline meters have also been installed downstream from the toll plazas at two other San Francisco Bay crossings, the San Mateo Bridge and the Dumbarton Bridge. However, these mainline meters have not yet been activated (as of September 2006). A mainline meter also exists on California State Route 125 southbound at its junction with Interstate 8 in La Mesa, California.
Ramp metering in Europe