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Traffic Light Synchronization --
An Air Quality Benefit, or a Sop for Motorists?

by Michael J. Vandeman, Ph.D.

California cities have been spending millions of dollars to synchronize their traffic lights ("signals", purists call them), so that motorists won't be stuck idling at red lights. The justification given is that the vehicles pollute (and consume fuel) most in stop-and-go traffic, and least in steady (optimally, 35 MPH) traffic. This seems plausible, but when transportation activist Mike Vandeman dug into the research behind it, he found that this reasoning was not justified. As a result, the Club decided to oppose the synchronization of traffic signals, preferring instead to spend scarce air quality funds on measures that are more likely to be truly beneficial.

How did so many public officials get misled? Easy: when a proposal sounds reasonable, and is at the same time extremely popular, scientific accuracy is often forgotten. In this case, none of the researchers considered the possibility that making it easier to drive might cause people to drive farther and more often, cancelling the alleged benefits. They were apparently so eager to help their fellow motorists (and perhaps profit from acting as consultants?), that they neglected to apply strict scientific standards to this research.

Everybody knows that a motor vehicle pollutes more in stop- and-go traffic than in smooth-flowing traffic. This fact has been used to justify expanding freeways, synchronizing traffic signals, and a multitude of other measures to speed up traffic. However, not too many people know under what conditions it is true, and when generalization to other circumstances is not justified.

First of all, the optimal speed for fuel consumption and emissions has been found to be between 25 and 35 MPH by the Air Resources Board, and 35 MPH by Newman and Kenworthy (transportation researchers from Perth, Australia). Thus, all of the freeway expansion projects in the Bay Area, which were promoted on the basis of speeding up traffic to 55 MPH, are of questionable value, as are synchronization projects where the speed objective is above 35.

Second, and more important, it is not valid to generalize from a single vehicle on a single occasion to a whole street full of vehicles over a long period. Newman and Kenworthy demonstrated why congestion relief in the form of roadway expansion actually worsens emissions and fuel consumption: although an individual vehicle may benefit, that effect is far outweighed by the fact that making traffic flow freely encourages people to drive farther and more often and makes it much less likely that they will choose to travel via public transit, bicycling, etc. In other words, highway expansion doesn't simply speed up individual vehicles, leaving the number of trips and VMT constant. If it did, it would be beneficial.

Similarly, synchronizing traffic signals doesn't just speed up existing trips. By making it easier for people to make long trips by automobile (while providing no benefit, or negative benefit, for bicycles and buses), could cause an increase in trips and VMT that would outweigh the alleged benefits. Since the Bay Area Air Quality Management District has been awarding millions of dollars to various cities to carry out synchronization projects, one would think that it would be interested in whether these projects actually are providing the air quality benefit that the law intended.

For that reason, Mike called Mr. David Marshall of the BAAQMD to find out how they justify these awards. He said that they make the decision based on modelling the data supplied by the project sponsors. When asked about the possibility of a growth-inducing effect, he said that they accept the judgment of the sponsor. In other words, if the city says the project will be beneficial (and not induce more traffic), then they get the money!

What about the scientific literature? Is there some support there? Mike examined all the literature in the Institute for Transportrtion Studies at the University of California, Berkeley, on this subject. Most of the "research" consists of modelling runs, using a program like TRANSYT (Traffic Network Study Tool). This proves nothing, because TRANSYT and the other programs assume that reducing congestion improves air quality and fuel consumption. Most of the remaining "research" didn't actually measure either emissions or fuel consumption, but assumed those benefits from other factors such as reducing the number of stops or increasing the average speed. And finally, of the extremely rare "research" that actually measured fuel consumption, none of the studies examined the possibility of a growth-inducing effect.

The bottom line is that there is not one shred of proof that traffic signal synchronization is beneficial (or that it is not)! And certainly not enough justification for spending millions of dollars on it, when there are many other uses that have an obvious air quality benefit (e.g. instituting a large gas tax).

Mike: "Having earned a Ph.D., I know that doing valid research is expensive and difficult. However, it is the only way we know of to arrive at reliable knowledge. Of course you still have to make decisions, while waiting for that research to be done, but it makes no sense to waste precious air quality funds (which we all know are hard to obtain!) on projects of dubious value.

"The real reason for the popularity of these projects is obvious: drivers love getting to their destinations faster. But it is about time we stop catering to automobile users, especially in the guise of a (false) concern for air quality. Let's leave the 'engineering solutions' (maximizing vehicle throughput, with no regard for the effect on the environment) to the engineers."

Michael J. Vandeman, Ph.D.

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