Ramp meter

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A ramp meter or metering light is a device, usually a basic traffic light or a two-phase (red and green, no yellow) light, that regulates the flow of traffic entering freeways according to current traffic conditions. While numerous studies exist suggesting meters control congestion, all of them have been prepared by parties who are associated with the transportation and road building industry. State DOTs, FHWA, transportation consultants, engineering firms, and university civil engineering departments are the only sources. Funding for university research is supplied by DOT's.

WHAT CAUSES CONGESTION?

Since the goal of ramp metering is to control congestion it is important to understand what causes recurrent rush period congestion. The answer is simple. Any road can only carry a finite number of vehicles. If numbers greater than the free flow volume of any road seek to occupy that road at any time, speeds fall and like-wise the volume of traffic flowing diminishes. The speed/volume relationship associated with freeways is widely known and can be found in traffic literature and textbooks. An excellent source for actual volume/speed graphs from a real freeway is the UC-Berkeley I-80 website[link]. Maximum short term flow on freeways is about 2000 vehicles per hour per lane (vplph) at about 50 mph. The maximum possible sustainable flow on an average freeway is about 1800 vplph at about 40-45 mph. This corresponds to a vehicle density of 40 vehicles per lane mile. So if the demand rate results in sustained volumes greater than 40 vehicles per lane mile, flow breaks down and typically falls to 20-30 mph with volumes of 1200-1400 vplph. See Traffic Flow Fundamentals by Adolf May, Prentice-Hall, 1990, for an excellent description of the progression into congested flow and back into free flow on a freeway as well as graphs of density vs. speed and volume. Now, considering that the one and only cause of congestion is demand exceeding capacity (greater than 1800 vplph at 45 mph), it can be seen that the only possible remedies are to change demand or change capacity. Capacity is a function of the physical freeway itself (i.e., how many lanes). Ramp meters do not add lanes, ergo to modify congestion, ramp meters must change demand.

Ramp meters are claimed to reduce congestion (increase speed and volume) on freeways in a few ways.

1) Reduction of Demand

2) Platoon Breakup

3) Diversion

REDUCTION of DEMAND

The concept of demand reduction is that ramp meters are able to restrict the total flow entering the freeway so it does not exceed the freeway's capacity (capacity of an average freeway is 2000 vehicles per lane per hour), thereby preventing congestion. Since all recurrent freeway congestion is caused by demand exceeding capacity, by definition ramp meters must change demand if they are to modify congestion (again see Traffic Flow Fundamentals by Adolf May, Prentice-Hall, 1990, for verification of this fact as well as fundamental freeway flow theory and graphs). On actual large city freeways demand can exceed capacity for 1 to 3 hours per rush period with average ramp demands of 600 vehcles per hour. An average ramp can store between 20 and 80 vehicles. These ramps will fill in fractions of an hour (e.g., 40 vehicles queued / 600 vehicles per hour = 0.067 hr = 4.0 minutes) at which point they become steady-state feeders in which release rate equals demand rate. Otherwise, the number of queued vehicles would build throughout the rush period and spill out onto arterial streets. In fact, metered ramps contain queue loop detectors which signal the controller that the vehicle queue is approaching the ramp's capacity. The controller then increases the release rate to reduce the ramp queue regardless of mainline conditions. This cyclical filling and emptying of the ramp creates a delay buffer for queued drivers but does not change demand rate after the initial 2 to 15 minute fill period just after the meters are activated. As discussed above, no change in demand rate equals no change in congestion. Claims aside, studies asserting the effectiveness of ramp meters never present data showing demand rate changes caused by ramp meters. If ramp meters truly could keep demand rate below capacity there would be no congestion on the freeways of the 25 to 30 U.S. Metropolitan areas which have installed ramp meters. All of these systems continue to be congested every weekday.

PLATOON BREAKUP

A Platoon is a group of vehicles traveling in close proximity. For example, a group released by an arterial traffic signal changing from red to green. The objective of this technique is to break up platoons of vehicles entering freeways, ensuring that traffic can merge more easily. Platoon breakup is a valid traffic management tool, but this method does not require the creation of steady-state queues on ramps. A ramp meter cycle rate of about 3 seconds per vehicle is adequate in most cases to break up any platoons while creating only transient queues so that driver time loss is minimized. It should be noted that platoon breakup techniques are only useful on ramps that experience platoons. Its use in other situations is counter-productive since motorists are delayed while receiving no benefit and in some cases with an increaed risk because they must accelerate more quickly over a shorter distance from a dead stop at the meter to the merge point. Notice that platoon breakup is not a modifier of demand rate and is therefore incapable of affecting congestion. If it were, there would be no congestion on the freeways of the 25 to 30 U.S. Metropolitan areas which have installed ramp meters. All of these systems continue to be congested every weekday even though all platooning has been eliminated.

DIVERSION

The premise of diversion is that ramp delay will cause some drivers to choose other routes thereby reducing demand. When considering this concept it is instructive to refer to "Wardrop's User Optimum (1952)" which states that any traffic network is at equilibrium. In other words, each driver has selected their own optimal route (every individual’s personal experience verifies this fact). Any alternate route has already been judged inferior. If it were not inferior, it would have been selected as the primary route. If ramp meters cause drivers to go elsewhere, they will pay a price in lost time, increased crash risk (city streets experience three times more crashes per vehicle mile driven than freeways), and increased fuel use. Meanwhile, a queue of drivers must be maintained to create the desired diversion. All these queued drivers lose time, waste fuel, and create unnecessary emissions. Simply put, any effort to modify the patterns of the transportation grid via ramp meters can only result in a less efficient total system since alternate routes are by definition inferior. Published studies such as the Twin Cities 2000 Meter Shutdown discussed below have reported no appreciable diversion when ramp meters are active.

The MINNEAPOLIS 2000 METER SHUT DOWN

In 2000, a $650,000 [experiment] was mandated by the Minnesota State Legislature in response to citizen complaints. The study involved shutting off all 433 ramp meters in the Minneapolis-St. Paul area for eight weeks to test their effectiveness. The study's authors claim that the Minneapolis ramp meters reduce accidents and marginally reduce total travel time compared with the unmetered case. However, they remain controversial, and the Minnesota State Department of Transportation has developed new, less onerous 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, and timing has been altered so that no driver waits more than 4 minutes in ramp queue. Before the shutdown, Minneapolis ramps were unique in that ramp queue loop detectors were not employed to keep lines of metered cars from stretching back onto arterial streets. In some cases MnDOT required drivers to wait as long as 15 to 20 minutes to access the freeway. The 2000 study's claims suggest that ramp meters in Minneapolis provide a benefit greater than their cost. However, utilizing the data contained in Table 5.1 (p. 5-2)[link] of the report, and the calculational methods supplied by the study's author (Cambridge Systematics)[link], there is a net annual lost time cost to drivers of $5.0 million in the four freeway segments studied. While there certainly are additional costs in fuel waste and unnecessary emissions while idling on ramps, no estimation of these was included in the study. Interestingly, while the Twin Cities freeway system has sensors (loop detectors) which measure speed and volume on all segments 24 hours a day at about 1/2 mile intervals, the study looked at only four segments using "floating car" runs to measure speeds. Based on the "Run Log", these floating car runs covered at most 7.5% of the available 700 hrs of "with meters" and 8.5% of the available 700 hours of "without meters" during the study period. Collecting loop detector speed/volume data from the entire system for the full 700 hours of each condition would have been a more valid scientific approach.

RAMP METER 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 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.

Ramp Metering in North America

In the Los Angeles, Phoenix, and Minneapolis-St. Paul metropolitan areas they are commonplace, and they are found in more than two dozen smaller metropolitan areas.

Ramp metering was first implemented in 1963 on the Eisenhower Expressway (Interstate 290) in Chicago, Illinois. 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, California, San Diego, California, Sacramento, California, the San Francisco Bay Area, Minneapolis-St. Paul, Minnesota, Seattle, Washington, Denver, Colorado, Madison, Wisconsin, Atlanta, Georgia, Phoenix, Arizona, Salt Lake City, Utah, Portland, Oregon, Washington, DC (only along Interstate 395), Interstate 66 in Arlington County, Virginia, and Toronto, Ontario.

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

A mainline meter, which is a specialized form of metering, has been implemented at the San Francisco-Oakland Bay Bridge toll plaza. This meter was installed in the 1970s. Similar meters have also been istalled downstream from the toll plazas at two other San Francisco Bay crossings, the San Mateo Bridge and the Dumbarton Bridge.

Ramp metering in Ramp metering is used to regulate access to a number of major roads in Sydney, including:
M4 Western Motorway (Wallgrove Road on-ramp)
M5 East motorway (Kingsgrove Road on-ramp).
Citywest Link (to Anzac Bridge)
Ramp metering is also used on freeways in Melbourne, including:
the Eastern Freeway
the Monash Freeway
On most freeways, ramp metering is activated when sensors indicate that traffic is heavy, however, some freeways without sensors use time-based activation.

External links

[M5 Merge Safe (RTA)]

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