Ramp meter
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A ramp meter, ramp signal or metering light is a device, usually a basic traffic light or a two-phase (red and green, no yellow) light together with a signal controller, that regulates the flow of traffic entering freeways according to current traffic conditions.
Ramp meters are claimed to reduce congestion (increase speed and volume) on freeways by reducing demand and by breaking up platoons of cars. Two variations of demand reduction are commonly cited; one being access rate, the other diversion.[1]
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[edit] Intended effect
Ramp meters are installed to restrict the total flow entering the freeway, temporarily storing it on the ramps, a process called "access rate reduction." In this way, the traffic flow does not exceed the freeway's capacity. Another rationale for installing ramp meters is the argument that they prevent congestion and break up "platoons" of cars. A platoon is a group of vehicles traveling in close proximity, such as a group released by an arterial traffic signal changing from red to green. Advocates of ramp meters claim that they break up platoons of vehicles entering freeways, ensuring that traffic can merge more easily. Another premise of ramp meters is diversion. The delay caused by the ramp meter waiting period may cause some drivers to choose other routes thereby reducing demand for the freeway.[2]
[edit] 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 vehicles at the front of the line. 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.
[edit] 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 carriageway 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.
[edit] 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.
[edit] Ramp metering in North America
Ramp meters are commonplace in the New York City, Los Angeles, San Francisco, Chicago, Seattle, Phoenix, Columbus, Ohio, 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."
Ramp metering was first implemented in 1963 on the Eisenhower Expressway (Interstate 290) in Chicago. 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. Since then ramp-meters have been systematically deployed in many urban areas including Los Angeles; San Diego; Sacramento; the San Francisco Bay Area; Seattle; Denver; Phoenix; Las Vegas; Salt Lake City, Utah; Portland, Oregon; Minneapolis-St. Paul; Milwaukee, Wisconsin; Columbus, Ohio; Houston; Atlanta; Miami; Washington, DC (only along Interstate 395 and Interstate 66 in Arlington County, Virginia); and the Queen Elizabeth Way in Mississauga, Ontario.
Ramp meters have been withdrawn after initial introduction in several cities, including Austin, Texas; Dallas; and San Antonio, Texas. Disused metering signals can still be found along some parkways surrounding New York City and Detroit, as well as on one ramp to Interstate 64/U.S. Route 40 in St. Louis, MO. Although deactivated shortly after they were added, ramp meters have been reactivated at select interchanges of Interstate 476 in suburban Philadelphia.
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 QEW.
[edit] 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 [1]. 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 Sytematics and concluded that when the ramp meters were turned off freeway volume 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.
[edit] 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). Beaverton, Oregon has a mainline meter on the Cedar Hills Connector turning from Oregon Route 217 northbound to U.S. Route 26 eastbound.
[edit] Ramp metering in Europe
Ramp metering has been installed in several countries in Europe, including the United Kingdom, Germany and the Netherlands. A research project Euramp on ramp metering funded by the European Union is due for completion in March 2007.
[edit] United Kingdom
Ramp metering is being introduced widely in the UK after a pilot study by the Highways Agency which concluded that ramp metering provides a net benefit under certain conditions - generally more congested junctions, but is ineffective or provides a net disbenefit under other conditions.
[edit] The Netherlands
The first ramp metering in the Netherlands was introduced in 1989. Ramp metering is being introduced more widely in the Netherlands after a pilot study by the AVV Transport Research Centre which concluded that ramp metering can provide a small benefit for the traffic flow on the highway, leading to a higher capacity. Ramp meters can also contribute to decreasing 'rat running'. By 2006 50 ramp meters were installed. This number increases by 4 to 5 each year.
[edit] Ramp metering elsewhere
[edit] Japan
Ramp metering is being installed in Japan in the next few years to keep the flow of traffic moving in Japan. There are plans to install ramp meters on every on-ramp in the Japan motorway system.
[edit] Australia
Ramp metering is used to regulate access to a number of major roads in Sydney, including: M4 Western Motorway (Wallgrove Road on-ramp); the M5 East motorway (Kingsgrove Road on-ramp); and the citywest Link to Anzac Bridge. Ramp metering is also used on freeways in Melbourne, including the Eastern Freeway and the Monash Freeway. Brisbane's Pacific Motorway also uses Ramp metering on some on ramps. On most freeways, ramp metering is activated when sensors indicate that traffic is heavy, however, some freeways without sensors use time-based activation.
[edit] New Zealand
Ramp metering is being installed Auckland wide after a successful trial on Mahunga Drive, before the Mangere Bridge. It will be installed on 61 Southern, Northern and North Western Motorway on-ramps [2] due to thick merging behaviour on several on-ramps during peak times. It will also be installed on all of the CMJ Spaghetti Junctionlinks. Auckland's ramp signals feature an amber light in order not to confuse drivers as to the fact that normal traffic signal rules apply.
Signals in Auckland have found a 30km/h average speed increase while Ramp Signals are running, and are allowing 600-650 more vehicles through motorway sections. [3]
[edit] South Africa
Ramp meters have been installed on the Samrand South bound, Old Johannesburg South bound and on New Road North and South bound interchanges on the N1 Ben Schoeman highway. The ramp metering is part of the Intelligent Transport System launched in October 2007 to aid traffic flow between Johannesburg and Pretoria.
Has also been installed on the north bound on ramp from Blue Lagoon to the M4 Highway in Durban since early 2007
[edit] See also
[edit] References
[edit] External links
- M5 Merge Safe (RTA)
- University of Minnesota
- Ramp Meters - Gives an overview and lists various locations.
- Ramp Signalling - Auckland Ramp Signalling information
