August 09, 2005
For Clean Air, Work Downtown
In our ongoing quest to discover how land use and urban form links to human health effects, I recently stumbled across something odd. It's a 2000 study of vehicle emissions per household in Puget Sound, authored by Larry Frank. I wanted to find out if there is a connection between air pollution and urban density. According to this study, there is, but in a way I didn't expect.
It turns out that the strongest land-use correlate to low household emissions is not residential density, but job-site employment density. That is, from a statistical standpoint, it matters less whether you live on Capital Hill or the Sammamish Plateau than whether you work in downtown Seattle or Bothell. The difference, I suppose, is that downtown Seattle and other places with high employment density are well-served by transit and are generally easier to get to with lower vehicle emissions than more far-flung workplaces.
Interestingly--this is only interesting if you're a geek; otherwise skip to the next paragraph--the drop in household emissions does not observe a linear relationship with employment density. For the lowest three quartiles of employment density household emissions are about the same (they're a little higher in the lowest density quartile), but then they drop off sharply at the beginning of the highest density quartile. This suggests that there's a threshold of employment density--perhaps the density at which transit, carpooling, etc become viable--after which emissions drop quickly.
It's also interesting, I think, that in this study residential density is less strongly correlated with lower household emissions. There is still a correlation--higher residential densities meant less vehicle emissions--but the difference, while significant, was relatively minor.
One reason perhaps emerges in another set of correlations. This study found that households located in census tracts with high employment density, greater mixes of land-use, and greater street network density--in other words, places with many characteristics of city living--actually generate more vehicle trips and more vehicle trips with a cold engine (which produces a disproportionate share of tailpipe emissions). Probably, this is because there are more services and amenities nearby and there's less incentive to "chain" trips together as a typical suburban commuter might on the way to or from work. Even so, the higher density households produce fewer emissions simply because the trips are not as long as for households in lower densities.
There's a lesson here, maybe, for those of us interested in urban form as well as everyone who's interested in improving air quality. From a public health perspective, it may make more sense to concentrate jobs in dense nodes with good transit access than to worry about other land-use features. Maybe the best reform to reduce vehicle emissions is more office space downtown.
About the study: The study uses an exhaustive (heh, heh) methodology that calculates three types of emissions (NOx, CO, and VOC) that accounts not only for driving distance, but also for speed, travel time, and emissions from starting the car (adjusted for estimated engine temperature at start). Its findings are based on data from the Puget Sound Transportation Panel Travel Survey, which records travel for 1,700 households over a two-day period by giving each member of the household over 15 a diary for recording trips and their characteristics.
UPDATE 8/10/05: Here's a link to an abstract of the study. As far as I know, the full version is not online.
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Yesterday Eric de Place held forth over pollution and urban design on the Cascadia Scorecard site. Examination of a 2000 Larry Frank study of Puget Sound vehicle emissions per household (which he doesn't link to and we can't find - C'mon Eric, it's the... [Read More]
Tracked on Aug 10, 2005 1:28:00 PM
So, Eric, do the monitoring data show that downtown has lower PM, NOX and O3 than the suburbs?
It's great that we can see correlations between land use and air quality, but what role to urban canyons and vegetation play in concentrating pollution?
We must know the actual air quality data from monitoring to have a full picture of the issue.
Posted by: Dan Staley | Aug 10, 2005 1:53:34 PM
Right. We need actual monitoring data to get a full understanding of air quality. And, as you suggest, air quality in any specific locale is affected by vegetation, urban canyons, and other physical features -- even the time of day and weather.
Frank's study isn't based on air quality monitoring. But his study does allow us to characterize the amount of emissions produced by different households that participate in different kinds of land-uses. Or, as Frank himself puts it, the study "presents a methodology to link land use with vehicle emissions via travel survey data."
I think the evidence would say that, generally speaking, downtown air quality is worse than suburban air quality. Nevertheless, on average, households with a workplace in high employment density locations produce significantly fewer emissions than households with a workplace in lower employment density locations.
Posted by: Eric de Place | Aug 10, 2005 3:12:18 PM
You're right, Eric, about households and employment density, and it does point out our separation of live-work space and how living downtown may be beneficial, *if* our live-work spaces are proximate.
I, personally, like to be careful about touting a downtown existence, as the air quality in a downtown is likely worse than in the suburbs; there are tradeoffs to an urban existence and these tradeoffs should drive an urge to find solutions to the externalities of internal combustion transportation.
 viz: http://press.psprings.co.uk/jech/september/755_ch31674.pdf
and press release: http://www.newswise.com/articles/view/513689/
Posted by: Dan Staley | Aug 12, 2005 11:07:13 AM
Oh, BTW, the link to Frank's abstract (in the update) got cut off:
Posted by: Dan Staley | Aug 12, 2005 11:20:13 AM