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. 
But 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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high-ground mantle of environmentalism. “SAVE THE TREES!” “PROTECT BAMBI!” Many ecologists and environmentalists (who should know better) also get caught up in the NIMBY hysteria of stopping all development (even development that furthers community quality of life objectives). It has only been recently that the national Sierra Club has admirably reduced their widespread NIMBY efforts and focused more attention on the real culprit — sprawl. In my work as a town planner in Florida, there were several instances where in-town development (even in-town pedestrian paths, of all things!) were hammered by intelligent environmentalists — environmentalists who were comparatively silent in the face of the incremental, relentless, larger-scale ecological destruction that was happening in outlying areas (ironically, such sprawl development was happening at an accelerated pace in part because of the actions of in-town NIMBYism). I don’t necessarily hold up all proposed in-town infill developments as models of sustainable, walkable design. I do think, however, that such infill — in the grander scheme of environmental sustainability — is a much-preferred form of development than dispersed sprawl development because, crucially, the infill development is found in ecologically preferred locations. I much prefer the loss of, say, a few trees in urban, disturbed woodlands (that many environmentalists fight tooth and nail to save), or the loss of a few raccoons and squirrels (that some environmentalists also rally to protect), to the loss of hundreds of acres of nearly pristine woodlands, and high-quality habitat that is home to, say, eagles, fox squirrels, and gopher tortoise. I honestly don’t believe there is a third choice: Loss of neither in-town squirrels or outlying quality ecosystems (the choice that naively believes we can somehow stop population growth). I believe that south Florida is a testament to the belief that there was no third choice. That infamous region — and many others throughout the nation — shows the ruinous results of fighting against in-town, infill development to “save Bambi.” I continue to have little patience for the many otherwise intelligent environmentalists and neighborhood activists who shriek about their no-compromise position of minimizing residential densities. Such activists believe low-density development is the be-all-and-end-all of environmental conservation when it comes to in-town development. There are countless environmentalists who are guilty of this. There is little that I can think of that is a more damaging strategy for quality of life in our future than to persist in the strategy of thinking that low densities will save us. Environmentalists must get on board with the idea that we need higher, livable densities (or, to use a term that is less inflammatory, “more housing”) in proper locations. If this does not happen, we will have no chance of averting a south Florida future… Another way of putting this is that more housing (more residential density) is not the problem. No, the key to a future rich in sustainability, quality of life, transportation choice, and civic pride is to insist on modest sizes. Modestly sized street dimensions. Modest distances between homes, shops and jobs (and, implicitly, modest community and neighborhood size). Modest building setbacks. All of these modest sizes, as our most loved, charming cities show, are best achieved by providing more housing (more density), not less. By stark contrast, sprawl is most accurately defined by large sizes. Big setbacks, huge street dimensions. Monstrous setbacks. All of these undesirable features are mostly likely to be realized when NIMBYs fight for less housing. In other words, “large-size sprawl” is scaled for cars, not people. A deadly form of the disease afflicting most all cities in America. It is the disease of gigantism. _________________________________________________ Visit my urban design website read more about what I have to say on those topics. You can also schedule me to give a speech in your community about transportation and congestion, land use development and sprawl, and improving quality of life. Visit: 
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