Controlled Access Through Metered Ramps Could Free Up Our Clogged
by Robert Sanders
The thought of freeway gridlock strikes terror in the hearts of traffic engineers and commuters alike, but a recent analysis of the problem by a Berkeley engineer indicates that in many cases it can be avoided by a simple technique--ramp metering.
According to Carlos Daganzo, professor of civil engineering and a specialist in traffic flow, ring roads or closed-loop freeways--sometimes called beltways--have the greatest potential for gridlock.
His mathematical analysis of circular freeways, published in a January 1995 research report from the Institute of Transportation Studies, showed that if the congestion extends all the way around the loop, traffic flow can drop by half in less than an hour on a three-lane freeway, while speeds can drop by a factor of six.
"In reality traffic would never come to a complete halt because people would get off and take surface streets," Daganzo says. "But once traffic speed has dropped to the level of the surrounding streets--which can happen quite fast--you no longer have a functioning freeway."
The good news, he says, is that gridlock can be avoided and traffic flow speeded up by installing meters to control traffic moving from surface streets onto the freeway. Metering lights at freeway on ramps are used here and there around the country to control traffic entering the freeway, but their broad use has been stymied by the politically touchy issue of which ramps to meter.
"Although not a solution to all congestion problems, this has the potential for having an incredible impact on locations susceptible to gridlock," Daganzo says. "If a beltway is allowed to become congested everywhere for lack of metering, the result can be catastrophic. In cases like this, restricting flow on all ramps can be beneficial and equitable, since it's restricting everyone."
In cities with congested ring roads, such as Barcelona, Boston, Paris or Tokyo, he predicts as much as a 20 percent improvement in traffic flow.
Though Daganzo's conclusions come from a mathematical analysis of an idealized ring freeway--that is, one that loops around to encircle a city--the results apply equally to any freeway or system of freeways that form a closed loop, Daganzo says. And it applies also to freeways cutting through a city if traffic exits more or less equally from each ramp within the city.
The mathematical model has helped us understand how traffic moves, and it tells us qualitatively how to improve it," he says. "Already we know how to fix some things. Show me a traffic problem and I can tell you if it can be fixed by metering and which ramps to meter."
One simple rule of thumb that came out of his analysis, for example, is that incoming traffic from a ramp should never be allowed to create a backup extending past the previous exit. The worst situation is when a bottleneck creates a traffic queue that propagates all the way around the freeway loop, because this marks the onset of gridlock.
The key to preventing total gridlock is to start metering upstream on ramps before this situation arises.
Already many freeways have roadbed detectors that measure traffic flow--the number of cars per minute--as well as traffic density. That's all the information needed to determine when and where to turn on ramp meters, he says.
"Effective ramp metering should keep the freeway flowing close to its maximum rate without allowing queues to form," Daganzo says. "With information from the roadbed detectors it is possible to determine when queues begin to form, and this should allow us to adjust the metering rates."
The need for ways to alleviate congestion other than building new freeways is urgent. Most state agencies such as California's Department of Transportation--Caltrans--have no plans to build major new freeways because of the cost and associated social problems, such as environmental damage and community disruption.
Instead they are looking at alternatives such as ramp metering, high occupancy vehicle (HOV) lanes, changeable message signs and toll roads.
"The solution is not to build more freeways, but to make better use of those we have," Daganzo says.
While ramp meters are common in Southern California, in the region around San Francisco ramp metering is now used only in the South Bay around San Jose in Santa Clara County, where nearly 100 ramps are metered.
According to Caltrans senior transportation engineer Rod Oto, however, meters currently are being installed on 20 ramps along Interstate 880 in Hayward and San Leandro, and within two years the entire I-880 corridor from the Santa Clara County line to I-980 in Oakland will be metered.
Until now most decisions about where to use alternative traffic control measures have been made by traffic engineers based on their field experience because systematic analyses of the problem based on a realistic theory of traffic flow had not been made, Daganzo says.
"The theory hasn't been developed until now, so practice was based on trial and error, and experience," he says. "These rules can be improved with a systematic analysis."