Ramp meter

Metered ramp on I-894.

A Portland, Oregon ramp meter.

A ramp meter, ramp signal or metering light is a device, usually a basic traffic light or a two-section signal (red and green only, no yellow) light together with a signal controller, that regulates the flow of traffic entering freeways according to current traffic conditions.It is the use of traffic signals at freeway on-ramps to manage the rate of automobiles entering the freeway. Ramp metering systems have proved to be successful in decreasing traffic and improving driver safety.

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.

Impacts

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 travelling in 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.

Criticisms of ramp metering primarily centre on impacts to users nearer to the urban core, which are typically delayed by the ramp metering, as compared to often uncontrolled users approaching from further out along the system. Such considerations may raise questions of equity, as was prevalent with the Milwaukee system. In the longer-term, it is conceivable that population growth may be encouraged further from the urban core and discouraged nearer, potentially encouraging sprawl development and increasing fuel usage. Additional concerns include queued traffic overflowing out of the ramps and onto other streets approaching the interchange. However some cities, like Atlanta, have fixed this by having the meters shut off temporarily to "flush out" the ramp once the traffic hits the streets. The meters then turn back on once the ramp has been cleared. Another concern is the increased pollutant emissions arising from the metered traffic and from the additional emissions from accelerating from a stop to freeway speeds (versus accelerating from street speeds to freeway speeds).

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 15ramps, 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.

The only existing integrated system in the U.S. is the INFORM project, but the concept is attracting considerable interest. Two cities, Seattle and Irvine, California, are in the process of implementing systems. Numerous other agencies are actively considering the integration of freeway and signal control systems. The initial efforts are primarily aimed at non-recurring situations where the signal timing can be modified in response to freeway incidents. Work is also underway, however, on corridor wide surveillance and adaptive control strategies.

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.

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. Since then ramp-meters have been systematically deployed in many urban areas including Los Angeles; San Diego; Sacramento; the San Francisco Bay Area; Fresno; Seattle; Denver; Phoenix; Las Vegas; Salt Lake City; Portland; Minneapolis-St. Paul; Milwaukee; Columbus; Cincinnati; Houston; Atlanta; Miami; Washington, DC (only along Interstate 395 and Interstate 66 in Arlington County, Virginia); Kansas City, Missouri; and along the Queen Elizabeth Way in Mississauga, Ontario, Canada.

A Milwaukee, Wisconsin ramp meter

Ramp meters are commonplace in the New York City, Los Angeles, San Francisco, Chicago, Seattle, Phoenix, Milwaukee, Columbus, 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, as well as on one ramp to Interstate 64/U.S. Route 40 in St. Louis, MO, on the WB entrance ramp from McCausland Ave., which will be removed as part of the New I-64 project in winter 2008. 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, and are scheduled to go into effect on November 24, 2009.

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 [3]. 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 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.

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. 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

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.

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.

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.

Germany

Ramp metering has been implemented on Autobahns in several areas in Germany, including the Ruhr area, Munich, and Hamburg.

Ramp metering elsewhere

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.

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 motorways, ramp metering is activated when sensors indicate that traffic is heavy, however, some motorways without sensors use time-based activation.

The recent M1 Upgrade in Melbourne has installed 62 ramp meters that are coordinated using the HERO suit of algorithms developed my Markos "The One" Papageorgiou and Associates from the Technical University of Crete. The system has been built on the STREAMS platform and utilises the state-of-the-art ITS architecture. All the ramps can be linked when required to resolve motorway bottlenecks before they emerge. The results of a recent trial improved capacity by 9% over the previous fixed time ramp metering system, average speeds increased by 20kmh and traffic throughput at bottleneck locations can be reliably maintained around 2200 PCE per lane (note the M1 is major freight route to the Port of Melbourne and has a very high heavy articulated vehicle mix. The HERO system takes real time data every 20 seconds from the motorway, ramps and arterial road in order determine the best signal timing for the next 20 seconds. The data detection system comprises Sensys detectors in every freeway lane at 500m spacings with a minimum detectors at three locations on each ramp including the freeway entrance with the arterial road. The system also manages the arterial road interface with the freeway, balances ramp queues and delays across ramps, and is capable of managing bottlenecks 3–4 km downstream of a ramp entrance. The system is also supplemented by real time travel time information to key destinations and incident and congestion information displayed on specially designed full colour VMS on the approaches to the freeway entrance ramps. This information provides sufficient advice for motorists to determine whether or not to use the freeway during incidents etc. The system also provides dynamic ramp closure in the event of a major incident A useful handbook on the design of coordinated freeway ramp metering is obtained from the following link. VicRoads Managed Freeways - Freeway Ramp Signals Handbook

New Zealand

Ramp signalling on North Western to Northern connection in Auckland.

Ramp metering is being installed Auckland-wide after a successful trial on Mahunga Drive, before the Mangere Bridge. It is progressively being installed on 61 Southern, Northern and North Western Motorway on-ramps due to thick merging behaviour on several on-ramps during peak times. It will also be installed on all of the links. Auckland's ramp signals feature an amber light to clarify that normal traffic signal rules apply. Drivers failing to obey a red ramp signal will be ticketed and fined as if they had violated a normal red traffic light.

The majority of motorway on-ramps which did not already feature two-lane merging are being extended to allow for two lanes to the ramp signal stop line, to reduce on-ramp queue congestion (for example, Panmure on-ramp northbound). Where two lanes apply, one vehicle per lane is released per green signal. Due to their constrained design, other on-ramps remain one-lane only despite being ramp-signalled (e.g. Symonds Street on-ramp southbound).

Electronic message boards indicate whether ramp signals apply. The first board will display "RAMP SIGNAL ON" when ramp signals are activated, and the second board (closer to the on-ramp) "PREPARE TO STOP". Where ramp signals are switched off, the first board is blank and the second board displays "RAMP SIGNAL OFF".

Signals in Auckland have found a 30 km/h average speed increase while Ramp Signals are running, and are allowing 600-650 more vehicles through motorway sections.

South Africa

Ramp meters were, for a while, 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 was 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

Taiwan

Freeways in Taiwan use ramp meters during peak hours since 1993. Traffic enforcement cameras are deployed to deter running the red lights, but a bus lane at Taipei Interchange from northbound Chongqing North Road to southbound National Highway No. 1 in northern Datong District, Taipei allows buses and properly indicated emergency vehicles to bypass the traffic control imposed by the ramp meters.

See also

1. Ramp Meter Design Manual

External links

• M5 Merge Safe (RTA)
• University of Minnesota
• Ramp Signalling - Auckland Ramp Signalling information
• Ramp Meters in the U.S.