Ramp Meter F.A.Q.

(Frequently Asked Questions)



Q: New freeway ramp meters and carpool lanes are being added at more and more locations. Why is this happening and why do we need to have traffic flow "managed"?

A: A lot of people have been asking us some tough questions about this. Overall, both ramp meters and carpool lanes are important traffic management techniques designed both to keep traffic flowing on the freeways and to reduce accidents. Intelligent management is necessary to preserve any of our precious resources -- including transportation. If our system of roads and freeways bogs down, the vital lifeblood of the Bay Area is endangered. With traffic congestion constantly increasing, sound management is a must.

Q: How do ramp meters -- or signals -- at freeway on-ramps help traffic flow?

A: By regulating the flow of traffic entering the freeways during peak traffic hours, the overall flow of traffic on the freeways is smoother. The regulated flow means we can accommodate more vehicles per hour on the freeways, shorten commute times, and provide a higher degree of safety.

Q: Are all meters operated the same way? It seems as if there are different kinds in use on different ramps.

A: Yes, there are slight differences, and just as every freeway has differing characteristics, every on-ramp is also somewhat different, especially since it serves as the link between the surface streets and the freeway. Thus we need to "customize" different operational programs for each ramp so that each can be most responsive to actual traffic conditions at that location.

Whenever you use any metered on-ramp, there are two basic things to remember:

Q: But there seems to be so little consistency from ramp to ramp. In some cases, the metering hours are different; in other cases, there are major differences in red-green cycles. Why is this?

A: Again, it relates to the traffic patterns on the freeway in question. Meters normally operate during the morning and afternoon commute hours. On some freeways, the peak traffic simply "surges" at a different time than others. Our traffic studies show that where the afternoon rush "hour" begins at 3 PM on one freeway, it might be 3:30 or 4 PM on another. We only want to regulate flow during these periods of intense use.

The red-green cycles do vary from ramp to ramp. While the green light is on an average of 2 seconds, the red light may vary anywhere from two to 13 seconds, depending upon traffic flow conditions.

Q: Is there any scientific basis for differing time cycles at ramp meters?

A: Yes, in many cases, they are set times, based on past traffic experience. Meters automatically turn themselves on at the beginning of the commute period and run at set cycles until the meters go off at the end of the commute period.

All ramp meters are responsive to traffic conditions, whether mechanically or electronically controlled. Our most sophisticated installations have a "demand responsive" system, where red-green cycle is actually controlled by the flow of traffic at that moment. We do this by computer with sensors embedded in the pavement. If the sensors tell the computer that freeway traffic is light, then the computer "tells" the meter to speed up and let cars enter the freeway as quickly as possible. If cars are moving slowly on the freeway, then the cycle is slowed, and on-ramp delay may be few seconds more per car. These sophisticated demand responsive meters are currently in use on a number of Bay Area freeways.

Q: It's nice to talk about a short delay on the metered on-ramps, but that is open to question. It seems like the wait is often several minutes. Why?

A: Waiting time obviously varies depending upon how many cars are ahead of you on the ramp. In the slowest situation -- a 13 second red and a two second green cycle -- 4 cars a minute would be allowed to enter on a specific lane; in the fastest situation 15 cars a minute could enter.

But you have to remember that if you're traveling any appreciable distance at all, that you gain back this time -- and more -- with faster speeds on the freeway itself. The trick, of course, is to know when to use the freeway and when to use the surface streets. While it varies from area to area and time to time, if you're only traveling a short distance during the commute period, it may be just as fast (or faster) to go by surface street. Also, some metered on-ramps are more lightly used than others, and the wait is less.

Q: Sometimes it doesn't seem like the freeways are any faster with the meters on. How do you know you save time?

A: Meters have been in use in the Bay Area since 1971. Over the years, every project has been subjected to exhaustive "before and after" tests, using electronic counters, personal monitors, in-pavement sensors, computer studies, and even aerial photography. Average speeds do increase, and sometimes rather substantially. For example, new ramp meters were activated in June, 1992 on northbound Route 101 between Bernal Road and Capitol Expressway. Traffic moved at an average 36 miles per hour before metering, and 44 miles per hour after. In April, 1993, ramp meters on southbound Route 280 between Route 85 and Winchester Boulevard improved the average speed to 51 miles per hour compared to an average 27 miles per hour before metering. Northbound ramp meters were activated in January, 1994 on the new Route 87 freeway from Route 85 to Route 280. Before metering, northbound traffic crawled at an average 15 miles per hour, and 57 miles per hour after. All of these evaluations were made during peak hours.

Keep in mind that since all of these traffic management systems have been introduced, the number of cars and drivers has also been increasing and this is a continuing problem. But remember that we can speed up traffic at the same time demand is growing. Think of what the freeways would be like without any controls!

Q: Well, it still seems like congestion is increasing during rush hours, and at locations where these new meters are supposed to help traffic flow faster. Isn't this a hole in your argument?

A: Overall, congestion is constantly growing. But this type of congestion mentioned is a peculiar problem, and one that we call "bottlenecking". Whenever traffic enters a freeway -- and then shifts left from lane to lane -- you create a slowing pattern behind that entrance zone. This is true with or without meters. On balance, overall congestion would be far worse on the freeway system without the metering program.

Q: Why do you claim that ramp metering is safer?

A: We've found that most freeway accidents occur during stop-and-go traffic conditions by inattentive drivers. Ramp metering provides a smoother flow of traffic which minimizes the problem. Generally speaking, freeway accidents have been measurably reduced according to before and after studies. For example, in the two years that the previously-mentioned northbound Route 101 meters have been in operation, overall accidents rates are about 33% lower than the two-year period before metering was implemented. Even more extraordinary is that accidents during the morning commute period have been reduced by over 50%.

On the Route 280 project, overall accidents rates have been reduced by about 14% after the metering was activated. During the afternoon commute period, accidents rates are 28% lower after the meters were turned on.

Q: With all the concerns about energy usage and air quality, it would seem to be very wasteful -- and polluting -- to have all these cars just sitting on the on-ramps. Is this true?

A: No, it's not. Actually, both energy and air quality are somewhat improved by maintaining faster freeway flow. While you obviously burn some gas while waiting on a metered ramp, your average speed is increased on the freeway itself. Most significantly, the incidence of stop-and-go driving is reduced on the freeway, and this is what really burns up most of the gasoline.

In terms of air quality, environmental experts have concluded that this program is slightly reducing quantities of nitrous oxide, carbon monoxide, and reactive hydrocarbons -- all components pollutants in smog. Again, this is a trade-off: there may not be any measurable differences on the on-ramps themselves, but with decreased freeway stop-and-go driving, and smoother flow, the overall air quality actually improves.

Q: What about the impact of ramp metering on adjacent city streets? Doesn't this program create severe local congestion?

A: There are two issues here. The first is a back-up of traffic waiting to enter at an on-ramp and the second is the possibility of increased general traffic diverted to surface streets. There is often a bit of confusion during the first few days of all metering projects, which does cause a bit more traffic congestion near the ramps. However, as commuters become accustomed to the meters and adjust their travel patterns, this temporary congestion disappears. In cases where traffic continues to back up at the ramp, most problems can be handled by adjusting the metering signal timing or by making minor improvements to the ramp or street to provide additional storage area.

In those cases where traffic has increased on city streets, the volume has not been significant enough to impact the operation of these streets. Sometimes the addition of meters coincides with widening of the freeway itself, which increases the capacity and eliminates the "bottlenecks", thus easing the traffic flow on nearby surface streets.

Q: Most of the meters seem to be placed on the "close in" ramps, which slows down our access to the freeways. It seems that we are being discriminated against in favor of those long distance commuters who do not have to deal with meters in distant communities.

A: Meters are placed on ramps which handle the highest volume of traffic during peak hours, and most of these are in the central area. Remember that the metering program saves overall time -- allowing faster speeds once you get on the freeway -- and in conjunction with savings in fuel and air quality, makes the trip faster and safer for all freeway users, whether or not they enter via a metered on-ramp.

Q: Separate carpool lanes seem to be popping up at more and more metered on-ramps. Why do we need carpool lanes?

A: Some ramps are metered and some metered ramps also feature a carpool lane. The purpose of this special lane is to provide a faster access for vehicles with two or more persons. This encourages ridesharing, by providing a special benefit for those people who carpool, vanpool, or ride the bus. The more people carpool, the fewer cars there are on the road and the better it is for everyone.

Q: But why should the two or more vehicles have such preferential treatment at metered on-ramps with carpool lanes? Doesn't that discriminate against those of us who can't rideshare for one reason or another?

A: The carpool lane helps every freeway user. Everyone's objective is to get on the freeway as quickly as possible. If you're driving alone and two of the five cars ahead of you are carpools, you would just as soon get them out of the way -- in the carpool lane. Then you would only have three ahead of you. That means faster travel time for everyone. Overall, total "person delay" is reduced by 30 percent.

Q: Is there a way to achieve total consistency in the ramp metering program?

A: It's difficult to entirely standardize such a system in an area so diverse as the Bay Area. We are constantly finding it necessary to readjust and fine tune the system as conditions change. In some cases, for instance, the traffic has been so backed up on the on-ramps and connecting surface streets that we've had to change the rules to let two cars pass on each green cycle. Otherwise, we'd bog down, and the purpose of a sound program is to head that off. The trick for each ramp is to read the signs.

Q: What about all those people who violate the metered on-ramp rules? It seems that there are quite a few.

A: Contrary to what you may think, we've experienced few problems on the metered on-ramps. After all, a red light is a red light, and everyone understands that red means stop and green means go. Most problems occur at the carpool lanes. Usually this happens when a single occupant auto gets into the carpool lane to avoid waiting. This is plainly against the law, and there will be increased Highway Patrol enforcement to curb these violators. It usually gets waiting drivers very upset and agitated, not unlike watching someone cut ahead of you in a theater line.

Violating on-ramp rules ends up being counter-productive, since every freeway lane has a certain finite capacity. Just like the proverbial straw that breaks the camel's back, one additional car -- a single auto in the wrong place at the wrong time -- has the potential of slowing an entire freeway. Interestingly, we've found that a freeway lane can carry the most cars per hour (2000) at a steady 40 miles per hour. Due to spacing required between cars at higher speeds -- for safety reasons -- only about 1700 cars per hour can move at 55. At slower speeds, such as 25 miles per hour, only about 1500 cars per hour can move in any given lane.

Q: Is installing meters expensive?

A: In terms of return on investment, no, since the accumulated savings to users yields dividends far beyond the minimal costs involved.

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