Is Traffic Signal Synchronization Justifiable?
Michael J. Vandeman
April 15, 1994
Claims that signal synchronization reduces emissions and saves fuel have been used to promote its use in numerous cities around the world. Millions of dollars have been spent to implement it. Traffic engineers use it to justify their employment. And yet these claims have never been scientifically demonstrated.
"Classical" traffic engineering has had as its goal the maximal throughput of vehicles, encouraging ever greater volumes and speeds of traffic. Recently, this goal has increasingly been questioned, since it has led to increases in noise, pollution, accidents, global warming, and, in general, a lower quality of life. Instead, traffic calming, and an emphasis on moving (or better still, not moving!) people and goods, rather than vehicles, is taking its place.
Traffic signal synchronization (TSS) is a good example of a tool that has received almost universal, and quite unjustified, allegiance. It is obviously extremely popular with motorists, but this is not sufficient justification for it, particularly since we need, for environmental and social reasons, to give preference to pedestrians, bicyclists, and transit users, and remove subsidies like TSS that only promote more auto use (cf. Hart, 1986). The fuel consumption question is particularly critical, since it affects CO2 levels, and hence, global warming (see Jaeger, 1988).
The usual justification given is that it allegedly reduces fuel consumption and emissions of air pollutants. But if you examine the "research", you find that there are no studies that directly measure either emissions or fuel consumption, especially area-wide, which is what counts! The benefits are inferred from the smoothing of traffic. The critical questions are begged, rather than tested. The computer programs used to predict emissions and fuel consumption (e.g. TRANSYT -- Traffic Network Study Tool, as in Brohard, 1986) contain within them the assumption that smoothing traffic flow reduces pollution and fuel usage. As we know from the work of Newman and Kenworthy (Murdoch University, Perth, Australia), facilitating traffic flow may be growth-inducing, and actually have the opposite effect (see Newman and Kenworthy, 1984; Kenworthy, Rainford, Newman, and Lyons, 1986;
Newman, Kenworthy, and Lyons, 1987 and 1988).
(For example, JHK & Associates, 1981, p.3-1: "Energy consumption and air quality improvements are assumed to closely track estimates of the above measures" ("Intersection delay, number of vehicle stops, vehicle travel time or inversely, vehicle speed") [emphasis added].)
There are also several other flaws in the research. There has been no attempt to assess the effects on reverse-flow and cross-flow traffic. There has been no measurement of effects on pedestrians, bicyclists, and transit users. If the "clean" modes of transport are impeded, the effect may well be detrimental. Obviously, it isn't possible to synchronize for both transit and automobiles, so optimizing for the latter causes less-than-optimal performance for transit.
There has been no quantitative consideration of the growth-inducing effects from encouraging long-distance auto travel at the expense of the other modes. If TSS gives people the feeling that they can travel quickly and easily, and therefore causes them to drive farther and more often, the fuel and pollution benefits can be negative. Or to quote Berg and Kaub (1985, p.24), "It was expected that fuel consumption savings would be attainable, however, this is not supported by the data.... Much of the benefit which would accrue to the arterial traffic flow is offset by increases in stops and delay to the cross-street traffic".
Deakin, May, and Skabardonis (1984, p.13-20) simply say: "When the Fuel Efficient Traffic Signal Management Program [FETSIM] was initiated, concerns were raised that the program might lead to induced traffic, which in turn could cancel out the traffic flow, fuel savings, and air quality benefits initially estimated [or even make them negative!]. Examination of the results of the 1983 program should lay these fears to rest. While in the aggregate, the travel time benefits of the program are large, from the perspective of the individual traveler they are too modest to be likely to induce mode shifts or additional trips. ... Thus, it seems safe to say that the benefits of the program will not be canceled out by program-induced traffic increases" [emphasis added]. Needless to say, this is wishful thinking, not research. Besides, the appropriate influencer is not actual time saved, but perceived time saved, which can be influenced by TSS hype.
Another consideration is financial. TSS is expensive to implement, in hardware, software, and personnel. It also requires continual maintenance, to respond to changing conditions. The timing mechanisms can't hold their accuracy forever, and whenever they lose it, the whole process of synchronization must be done over again at considerable additional expense (often with scarce "air quality" funds). After all, we aren't talking atomic clocks here!
In other words, there is absolutely no credible evidence that synchronization is beneficial to the community as a whole! Moreover, synchronizing for automobiles, and controlling signals to benefit the cleaner modes, are mutually exclusive: you can do one or the other, but not both. With so many clearly effective ways available to save energy and reduce air pollution (such as increasing fuel and parking charges), why should we waste millions of dollars on such a dubious device?
BERG, W. D. and KAUB, A. R. (1985) Case study evaluation of the safety and operational benefits of traffic signal coordination. Transportation Research Board, August.
BROHARD, T. (1986) Signal improvements save time and fuel. Public Works, February, 52-53.
DEAKIN, E. A., MAY, A. D. and SKABARDONIS, A. (1984) Energy savings from signal timing optimization: evaluation of California's statewide program. Compendium of Technical Papers. Institute of Transportation Engineers. Meeting (54th: S. F., CA), 13-16 - 13-20.
Hart, S. (1986). Huge city subsidies for autos, trucks. California Transit, July-Sept.
Jaeger, J. (1988). Developing policies for responding to climatic change. World Meteorological Organization, April.
JHK and ASSOCIATES (1981) The impact of traffic signal improvements on air quality and energy consumption. Technical Memorandum 1, February.
Kenworthy, J. R., Rainford, H., Newman, P. W. G. and Lyons, T. J. (1986) Fuel consumption, time saving and freeway speed limits. Traffic Engineering and Control, September.
Newman, P. W. G. and Kenworthy, J. R. (1984). The use and abuse of driving cycle research: clarifying the relationship between traffic congestion, energy and emissions. Transportation Quarterly, 38, 615-635.
Newman, P. W. G., Kenworthy, J. R. and LYONS, T. J. (1987) Transport energy conservation policies for Australian cities. End of Grant Report, Project No. 836, August.
Newman, P. W. G., Kenworthy, J. R. and LYONS, T. J. (1988) Does free-flowing traffic save energy and lower emissions in cities? Search, 19, No.5-6, September-November.