Does Traffic Calming Increase Air Pollution?

By Dom Nozzi

In May of 2000, a resident of Boulder, Colorado emailed a fairly common complaint to me in response to my praise of “traffic calming” (which uses street modifications to compel cars to slow down to safe, attentive speeds).

I thanked the gentleman for his comments. I went on to state that I had not spent a lot of time trying to track down every single report about traffic calming. But since I read a fair number of calming reports and have not seen a pollution problem reported, I’ve not had cause to doubt the claim of increased air pollution due to calming until I got his comments.

What I had learned from Jeff Kenworthy and Peter Newman, however, has made me highly skeptical of claims that suggest there is less air pollution from high-speed, free-flowing car traffic.

After all, it is too simplistic — too narrowly focused — to just think about the impacts of stop-and-start (or slow-and-start) car travel on air pollution emitted by individual motor vehicles. Yes, it is nearly certain that stop-and-start motor vehicle traffic increases air pollution emitted by individual cars in a highly localized, discreet location where the stopping and starting occurs. skycrest_2But this micro focus ignores the important but typically overlooked motorist behavior modification that occurs at the regional level when we widen streets or calm them. For example, how many trips are encouraged or discouraged (especially the low-priority car trips) when we widen a street or install traffic calming measures? How many more or less car trips occur at rush hour? How many more or less will drive instead of take transit, bicycle, or walk?

Kenworthy and Newman make the crucial point that travel behavior changes that we induce through widenings or calming on the scale of a community totally overwhelms any benefits of free-flowing traffic at the micro level of a given segment of street.

Consider the comparison between higher density congested areas and lower density, free-flowing areas. One would expect that the congested areas generate higher levels of air pollution than the free-flowing areas. But we know that people who live in higher density, more congested areas where transportation choice is high have been clearly shown to produce much less air pollution, per capita, and generate much less air pollution, per capita, than those who live in remote locations without a travel choice (those who have no choice but to travel by car). The worldwide analysis of cities conducted by Kenworthy and Newman confirms this.

Yes, on various congested street segments, air pollution is relatively high. But at the community-wide level, air pollution is much lower than cities with lower-density, free-flowing traffic. And this is because of the large reduction in the number of rush hour, major-street car trips that occur due to congestion, traffic calming, and other measures (“low-value” car trips that are induced when streets are widened or made more free flowing).

It is illogical to assume that making car travel easier with higher speed, free-flowing designs will reduce air pollution (and fuel consumption) impacts — given the likely behavior modification that induces motorists to engage in more driving than they would have engaged in had the street been more congested or more traffic calmed.

As Thomas Kuhn points out in The Structure of Scientific Revolutions, it is nearly impossible for those who have worked under the traditional paradigm to accept overwhelming evidence or conclusions from the new paradigm. For example, most of us will go to our graves steadfastly refusing to accept the premise that traffic congestion and traffic calming have a number of benefits, even though the evidence is mounting.


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Filed under Road Diet, Sprawl, Suburbia, Urban Design

2 responses to “Does Traffic Calming Increase Air Pollution?

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

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

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