Traffic Signal Synchronization – Why not?

I was having a debate over John Hilkevitch’s article a while back in the Tribune about synchronized traffic signals with a friend of mine. My friend was surprised that this technology, most impressively shown in the movie, The Italian Job, was not widely deployed in many major cities such as Chicago. Alas, we still rely on, in some cases, 1950s technology to power our traffic signals. The essence of his thinking was why had it taken so long to roll out this technology given its substantial benefits.

My response, colored by my experience (cynicism?) in transportation, was to be skeptical. Traffic engineering has long been full of promises on solving road congestion, with many solutions worse than the problem. When roads have been widened or new roads built, induced demand has been the result. If we make all of our signals “smart” and easy for people to drive in the city, what happens then? We’ll see a mode shift away from alternative transportation options as people discover that synchronized traffic signals make driving easier, thus increasing demand for driving. As Todd Litman notes,

Field tests 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 (Frey and Rouphail 2001).

Making driving easier brings all the negative externalities to the fore: increased auto emissions, gas consumption and pressure on land uses to accommodate cars. Thus, while traffic signal synchronization seems like a good idea, the unintended consequences are likely to make the existing traffic congestion status quo even worse.

Update: Dom Nozzi, making many of the same points, explains this in better detail here.


  1. Wanderer says

    Traffic signal synchronization also benefits bus operations. It makes bus travel faster and more reliable, and therefore more attractive to more passengers. How does that fit into the analysis?

    • says

      You are absolutely correct that traffic signal synchronization benefits bus operations. But we can improve bus operations without traffic signal synchronization. Rather, transit signal preemption, which is similar to what emergency vehicles use, will preempt the stoplight, either by holding a longer green or by making a red light at the cross street. Buses have used these devices along with street running trams and light rail vehicles to prioritize and make more efficient their operations when running in mixed traffic.

      Therefore, I think if we are going to invest in efficiency upgrades to speed up traffic in cities, why don’t we consider traffic signal preemption instead. This way we are not prioritizing cars where there is limited capacity, but rather transit, where capacity can be added without some of the negative externalities that cars bring.

      • TMW says

        If you’re so concerned about the potential of increased automobile traffic volumes simply by making traffic signals coordinated, maybe you should instead propose a vehicle restriction. It seems your sole intent is to make automobile travel so tedious and undesirable as to cause drivers not to drive. While this may appear to be a means to turn people on to transit, what is more likely is that a percentage of the drivers will simply avoid the route altogether and choose paths through neighborhoods and local streets… resulting in residents screaming for traffic calming measures due to all the cut-through traffic. You can’t have it all.

        • says

          In cities where there are alternatives to driving, yes, my intent is to make driving tedious and undesirable enough to shift mode share towards transit, bicycle, walking, etc. Keep in mind that driving alone has the highest mode share in every U.S. city not named New York. 
          According to 2009 ACS data, in Chicago, 61% of commuters 16 and older drive alone, despite a fairly robust transit system and high population density. Because our roads are already congested, I’d rather see more resources poured into other modes of transportation first. 

          There are other ways, of course to dissuade commuters from driving and those options should be explored. Parking pricing, fuel costs, congestion fees, registration and licensing fees, these are all potential tools that could be used to reduce driving mode share.

  2. Watcher says

    I’ve read the the actual report from Frey and Rouphail, two professors at North Carolina State University (NCSU), and their conclusions vastly differ from what was reported here.
    Here is an actual quote from the NCSU report, “The results of this study also suggest caution regarding the conventional wisdom about some traffic control measures, such as those aimed at “traffic calming”.  Measures such as frequent stop signs or speed bumps have the potential to increase real-world emissions because each occurrence of a stop sign or speed bump is also a likely location of accelerations.”
    So, in other words, not synchronizing signals can cause vehicles to excessively stop-and-go on an urban street.  Vehicles that spend a considerable amount of time accelerating emit more pollutants into the ambient atmosphere.  The main thrust of the NCSU report is that time in acceleration has more impact on emissions than time spent idling.
    I work in transportation research professionally, and I hate it when people read the conclusions from our reports and then lie about it.

    • Watcher says

      Furthermore, this quote from Frey and Rouphail is more telling, “NCDOT should be encouraged to carry out projects that reduce congestion on signalized arterials and/or improve coordination and signal timing as those strategies have been shown in this work to have a demonstrable effect on emission rate reduction.”

      • says

        The study that we are both referring to is here:

        I think we may be approaching the problem from different perspectives. I don’t disagree with you that acceleration increases emissions, as you (and the study) rightly point out. But, as the study also points out on page 282: 
        “NCDOT and other agencies should implement strategies such as improved signal timing and coordination, on corridors that are not bottlenecked by capacity problems, as a means to reduce emission rates on those corridors.”

        And in my post I am discussing cities, specifically Chicago, in which most (if not all) corridors are bottlenecked by capacity problems.

        My fear in cities is that traffic signal synchronization will cause induced demand. And while I understand that acceleration has more impact on emissions than idling, I’m more concerned that the induced demand generated from more efficient traffic flow will negate the benefits that traffic signal synchronization delivers, and not just on the emissions side, but also in terms of land use impacts, quality of life and alternative transportation modes.


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