Traffic Light Synchronization is NOT a Good Idea

By Dom Nozzi

Traffic light synchronization seems like a common sensical, no-brainer “solution” to relatively cheaply reduce air emissions and gas consumption. It seems so obvious that even highly intelligent progressives and environmentalists (nearly all of them) STRONGLY support this tactic. Michael Vandeman, in his article, “Traffic Light Synchronization — An Air Quality Benefit, or a Sop for Motorists?,” points out that many are misled when it comes to traffic light synchronization. “[W]hen a proposal sounds reasonable, and is at the same time extremely popular, scientific accuracy is often forgotten. In this case, none of the researchers [for synchronization] 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…that they neglected to apply strict scientific standards to this research.” Those who trumpet the alleged benefits of synchronization fail to take into account about the “induced demand” that Vandeman rightly points out. That is, they forget that the amount of travel by car is higher or lower based on the ease of car travel and the cost of car travel. The easier and cheaper we make car travel (As Jevons Paradox shows us. See:, the further people will travel by car and the more frequently that car travel will occur. In other words, if gas costs $20/gallon, people will drive less. If streets are congested or traffic signals force one to engage in start-and-stop driving, people will drive less. When people drive less, air pollution and gas consumption are reduced. Because traffic synchronization makes cars happier and thereby encourage more driving, air pollution and gas consumption are increased, not decreased (as most assume). Therefore, despite the overwhelming conventional wisdom, traffic synchronization does NOT reduce air emissions or gas consumption. Vandeman agrees when he points out that it seems like common sense that synchronization is a good idea. “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.” The big mistake, according to Vandeman, is to inappropriately generalize. “[I]t 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.” Todd Litman, of the Victoria Transport Policy Institute, adds a cautionary note. “Road widening and traffic signal synchronization are sometimes advocated as ways to reduce traffic congestion, and therefore energy consumption and pollution emissions. However, research suggests that at best these provide short-term reductions in energy use and emissions which are offset over the long-run due to Induced Travel. Field test indicate that shifting from congested to uncongested traffic conditions significantly reduces pollution emissions, but traffic signal synchronization on congested roads provides little measurable benefit, and can increase emissions in some situations. (See: This induced demand is the reason we cannot build (widen) our way out of congestion (because widening a road induces MORE car trips that would not have occurred had we not widened). If it is agreed that we cannot build our way out of congestion (or loosen our belt to solve obesity), why do some of us think we can synchronize our traffic lights out of congestion? The tragedy is that many communities — which have many vigorous advocates of synchronization, including “environmentalists” and other intelligent people who should know better — spend millions of public dollars to worsen transportation conditions, quality of life, air pollution, and gas consumption? In my view, proponents of synchronization are sadly wasting public dollars by moving communities further away from important community objectives. Traffic synchronization may be popular and may seem like an irrefutably good idea by most people, but the unintended consequences described above point out that synchronization is highly counterproductive to the objective of a more sustainable, pleasant community, less pollution, less gas consumption, and more transportation choices. _________________________________________________

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10 responses to “Traffic Light Synchronization is NOT a Good Idea

  1. I was googling around for an article I wrote on my blog:
    and saw that we are covering similar territory. I like your work.

  2. Thanks, Ryan. Nice article at the link you posted.

  3. Hannah Remtema

    Dom, does Vandeman have any recommendation as to how to estimate induced demand? They typical benefit/cost analysis for traffic signal optimization assigns a before and after dollar value to time and fuel (we measure changes emissions but do not assign it a dollar amount) based on field data – that’s how we decide how much we can congratulate ourselves. I certainly agree with the assertion that induced demand exists. But as a traffic engineer, I’m not sure how we account for it in that calculation.

    Certainly in some areas, the induced demand would be greater than others. If somehow lights could be timed so that driving in the city was really easy (and easy/cost of parking was taken out of the equation), I’m sure there would be biking, transit, and walking trips replaced. But what about a far-flung suburban arterial where all the trips are already over 5 miles and non-motorized/transit infrastructure doesn’t exist? We might be stealing trips from another parallel route, but that would be almost a wash for the overall network.

  4. Thanks for your thoughtful comments,Hannah. I don’t have any details about Vandeman’s work, and I’m not a traffic engineer so I don’t feel qualified to address your important questions. I’d say that if we don’t make driving easier (by synchronizing traffic lights), we will, over the long term, encourage some of those far-flung residents to live in closer-in locations. And even if that were not true, how would it be fair to those living in a more central location to pay taxes that make travel easier for those who chose to live in an unsustainable, car-dependent place? (by having them contribute to the dollars needed to synchronize lights).

  5. Pingback: The Many Transportation Reforms Needed in Boulder, Colorado | Dom's Plan B Blog

  6. Zong Tian

    I guess anyone can argue on anything from a different perspective. However, the argument presented here only reflects a glimpse of the overall issue. Public transit also needs to go through signals. Without well timed signals, how can you make the transit to be on time? If simply reducing travel is the primary objective, why don’t we get rid of the roads? Keep in mind that the majority of the trips, at least with the current social environment, are not leisure trips, but necessary trips for work and daily needs. How would the society evolve without them? How many of us are willing to go back to the Stone Age to live a simple life even though that is something we sometimes admire?

  7. Thanks for reading the blog. I very much agree w/ you that synchronizing signal lights for transit is helpful. I know that Portland OR did this in the past and may still do it. The data/studies I have seen indicate that a large number of car trips are lower value discretionary trips and that work trips are less than half of the trips. This is entirely what should be predicted since nearly all roads are untolled. “…why don’t we get rid of the roads?” is a straw man argument. No one I respect makes that recommendation. “…go back to the Stone Age…” is another straw man argument. In what sense does my essay suggest doing that? Going back to a primitive lifestyle is light years away from my suggestion that we do a better job of discouraging lower-value car trips.

  8. Pingback: Traffic Signal Synchronization - Why not? - Transport Nexus

  9. Pingback: Out-of-sync traffic signals worsen gridlock, but Miami-Dade says relief is on the way | Fort Lauderdale News

  10. Pingback: The Many Transportation Reforms Needed in Boulder, Colorado – Boulder Transportation Reforms

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