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Metered Ramp on I-894.

A Portland, Oregon ramp meter.

Ramp metering (RM) is a system that includes a ramp meter, ramp signal or metering light. A device, usually a basic three phase traffic light or a two-phase (red and green, no yellow) light together with a signal controller, regulates the flow of traffic entering motorwayss / freeways according to current traffic conditions.

Ramp metering is 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.

Intended effect

The principal aim of RM is to regulate flow from the slip road on to the main carriageway at a level that maximises through put on the main carriageway. An overview of the RM concept is provided below. Occupancy is the amount of time a vehicle detection loop is covered by a vehicle expressed as a percentage.

When traffic flow on the main carriageway is high this means that the traffic occupancy is also high. A high occupancy directly equates to smaller distances between vehicles or a low headway, in such conditions the road’s capacity is close to being reached, and small changes in the nature of the traffic flow cause it to become volatile and susceptible to flow breakdown. The introduction of traffic from the slip road can cause vehicles to change lanes and bunch leading to higher occupancy and lower headways. These shorter headways can be unsustainable at speed on the main carriageway; therefore for comfort and safety, drivers adjust their speed to account for the short stopping distances available.

Often this adjustment of headway causes following vehicles to brake, propagating a “wave” of braking vehicles in the traffic stream. Traffic occupancy in the wave becomes even higher. To compound the problem more vehicles enter the main carriageway from the slip road, thereby boosting occupancy even higher. If vehicles continue to join, ultimately the main carriageway speed drops to a point where flow breakdown occurs. In this situation vehicles are stopping at the back of a queue and then driving off the front of the queue. When flow breakdown is directly due to the merge, this stationary traffic can typically be seen between the merge area and approximately 2km downstream. Weather conditions, daylight, vehicle mix and gradients amongst other things can all affect the maximum throughput of any section of motorway.

To address this problem, RM aims to maximise throughput on the main carriageway without disrupting the local road network. It does this by controlling the discharge of traffic from the slip road to reduce the interference of merging traffic on the main line flow, thereby maintaining speeds at a higher level. Maintaining higher speeds postpones the onset and duration of flow breakdown on the main carriageway. To do this, the RM system relies on the measurement of traffic conditions on the main carriageway and attempts to maintain traffic on the road at ‘target occupancy’ by restricting the flow from the slip road.

Types

Some ramp metering installations have bypass lanes for high-occupancy vehicles, allowing carpoolers and buses to skip the queue and get directly on the carriageway. Systems often only operate in rush hour or other congested periods. Some installations have only one lane of traffic at the signal; others may have two or more lanes of traffic. In one common configuration, each entrance lane has two signals; a red-yellow-green signal mounted high for each lane, and another signal at driver height on a pole next to the stop line.

The overhead lights are for cars approaching the metering point; the low-mounted lights are intended to be used by the vehicles at the front of the line. In some variants of ramp metering, 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 and then only use red and green signals. In other variants, installations use red-yellow-green signals on both the upper and lower mounts on the pole, and operate in a standard green-yellow-red fashion.

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 May. 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.

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."

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.

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.

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.

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

Introduction

Ramp Metering (RM), was first introduced in the UK on the M6 near Birmingham in 1986 on the southbound access slip road at Junction 10. Its primary aim was to reduce congestion, which regularly disrupted main carriageway traffic flow south of the junction during morning peak periods. The congestion resulted from traffic flow breakdown at bottlenecks near the junctions. As a result of the successful implementation of RM at Junction 10 of the M6 the system was subsequently installed on the northbound slips at Junction 10, the southbound slips at M6 Junction 5 and 7 and at the north and southbound slips of M6 Junction 9 during 1988.

The results of monitoring during 1990 continued to indicate achievement of substantial benefits at Junction 10 southbound. The ramp metering system remained in operation until May 2000 when the equipment became obsolete and irreparable and consequently was switched off. With the continued development of RM outside the UK, and to confirm the benefits of the original UK installation, a new Ramp Metering Pilot Scheme (RMPS) was commissioned in 1998. To assess its operation and identify the factors that contribute to successful operation a ‘stand alone’ ramp metering system was required. This needed to be compatible with the English NMCS (National Motorway Communications Systems) network and suitable to develop national application notes and standards for the wider application of ramp metering on the road network.

In addition to the M6 J9 northbound and southbound and M6 J10 southbound sites, sites at M6 J7 southbound and M6 J5 southbound were upgraded to the new equipment as part of the RMPS scheme. A number of sites along Junctions 3, 11 and 12 of the M3 and Junctions 5, 7, 10 and 11 of the M27 were also selected for this trial. The RMPS became operational in August 2000 and included an extensive ‘before and after’ study to support the future business case. Extensive monitoring and data collection systems were developed, including traffic data collection, video monitoring and an automatic number plate recognition system. It was found that the M6 sites were successful in achieving benefits, whilst the M3/M27 sites showed mixed results with selected sites displaying modest and inconsistent benefits during certain time periods. These were shown as reduced journey times. With increasing levels of congestion across the UK motorways there is greater emphasis being placed on the need to manage the demand for travel and also to make the most efficient and effective use of the existing road network. As a consequence the Highways Agency (HA) undertook the necessary design, monitoring and evaluation work to implement RM at 30 sites (RM30) across the motorway network.

Ramp metering has been rolled out across the UK and a total of 85 sites are planned. Information on current progress can be obtained from http://www.highways.gov.uk/rampmetering.

Technical

A standalone ramp metering controller has been developed specifically for the UK implementation of the ramp metering concept. This, in addition to running specially designed software has been provided with remote configuration access and other means to communicate with traffic control systems and local authority Urban Traffic Control Centres.

Site Components

This section provides a brief description of the hardware and software requirements for RM implementation. The hardware includes all roadside installations and software includes the system algorithms for the ramp metering controller. The following paragraphs explain the main RM components.

Fixed Warning Signs

Warning signs are to be used, where applicable, to warn drivers of queues, and to alert drivers of the temporary traffic signals. Typical installations include ‘Queues Likely’ and ‘Part Time Signal’ signs located along the slip road.

High Friction Surfacing

High friction surfacing is typically required upstream of the stop line and on the lead up to the override loops. This anti-skid surface serves as a protection for the queue, and assists vehicles stopping for the signal heads at the stop line.

Road Markings

Road markings includes the installation of the stop line and modifications to the lane identifiers on the approach to the signals. Further road markings may include clarification of lane layout and modification to merge layout.

Signal Heads

Part time signal heads are used to regulate traffic flowing from the slip roads onto the motorway. The signal heads are fitted with a yellow backing board to distinguish them from standard traffic signals. The varying signal cycles are controlled by the ramp metering outstation.

Slip Road Traffic Sensors

Slip road loop detectors monitor traffic on the slip roads and regulate the queue formation. The slip road loops currently employed are one loop per lane per loop array. Slip road loops consist of:

• Release loops at the stop line, which indicate when a vehicle has left the stop line;
• Presence loops at the stop line, which indicate when a vehicle is present at the stop line;
• Queue override loops at the back of the vehicle storage area which indicate when a queue has reached the back of the storage area; and,
• Queue detection loops, which detect the length of the queue between the presence and override loops.

Upstream and Downstream Traffic Sensors (MIDAS Loops)

Upstream and downstream (of the merge) traffic is detected using the Motorway Incident Detection and Automatic Signalling (MIDAS) outstations. These outstations use arrays of induction loops in the motorway lanes to calculate speed, flow and other statistical information at the various sites, which is passed to the MIDAS subsystem.

Information on individual vehicles gathered by the MIDAS outstation is passed to the RMO via the MIDAS outstation’s ‘Outstation Auxiliary Link’ OAL, which the RMO processes into speed, flow and occupancy values.

Ramp Metering Outstation

The Ramp Metering Outstation (RMO) consists of the Ramp Metering Controller (RMC) and a Traffic Signal Controller (TSC) co-located in a nineteen inch rack within a cabinet. Also contained within the RMO cabinet is a communications modem.

The role of the RMO is to provide control functionality for the system. This is principally the calculation of the release rates and resultant variation the signal timings based on real time traffic data. This is achieved through the functionality of the RMC and TSC as follows:

The RMC is continuously gathering information from all of the various loop detectors (Slip road loops and MIDAS), and performing a series of ramp metering algorithms to calculate suitable release rates. The RMC uses RS485 ports to communicate with the MIDAS outstations, via the MIDAS outstation’s Outstation Auxiliary Link (OAL). The slip road loops are connected directly to loop interface cards on the RMC. The RMC performs data filtering of the slip road loop and OAL MIDAS data. The RMC then runs the algorithms (Using a range of site specific calibration parameters) to control the displayed signal sequences, in addition to other functionality including system logging, and fault reporting; and,

The TSC functionality includes providing control and power to the signal heads. This also includes determining red light failures, ambient light levels and preventing the display of incorrect aspect combinations. It also provides dimming of the signal heads in response to the levels of ambient light.

The RMO has a remote communications link, with suitable security arrangements, so that supervisory functions can be carried out remotely.

System Operation

The RMO operates in one of 4 operational modes listed below and in accordance with the system algorithm specification:

• Standby;
• Switching On State;
• Steady State; and,
• Switching Off State.

The RMO continually receives traffic data from the slip road loops and MIDAS outstations on the mainline. The system operation relies on this continuous stream of traffic data and varies the vehicle release flow over the stop line via the RM signal sequences.

In standby mode, the RMO continuously collects, filters and analyses input data from all traffic detectors and regularly evaluates the switch on/off algorithm. When preset levels are met the system switches to the ‘switching on state’ where it displays its first signal aspect. This is full green for a minimum period set during calibration, to indicate to the road user the activation of the system.

Upon completion of the ‘switching on’ state, the RMO enters normal operation which is the ‘steady state’ condition. In steady state operation, the RMO continues to collect, filter and analyse input and also regularly evaluates all algorithms in order to activate the signal aspects accordingly.

When the main carriageway traffic flows are returning to normal the RMO switches to ‘switching off’ state. In this state the RMO gradually increases the number of vehicles released from the stop line to dissipate any stored traffic in a safe and controlled manner. The RMO then returns to ‘standby’, deactivating the signals with a prolonged green period.

Steady State Algorithms

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.

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

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 will be 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 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 .

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

See also

1. Ramp Meter Design Manual

References

University of Minnesota

Ramp Meters

Ramp Signalling: http://www.aucklandmotorways.co.nz/cmjcompletepages/cmjcomplete.html CMJ Spaghetti Junction

External links

• M5 Merge Safe (RTA)
• University of Minnesota
• Ramp Meters - Gives an overview and lists various locations.
• Ramp Signalling - Auckland Ramp Signalling information

Category:Road transport