While it’s intuitive that sprawl leads to more miles traveled by car, solutions to the problem are somewhat less instinctive. Should we continue the push toward mixed-use development, encourage dense developments or build up near existing city centers?
A meta-analysis published recently in the Journal of the American Planning Association culls research from dozens of studies to develop a road map for planners and policy makers hoping to minimize our dependence on vehicles. I spoke last week with Reid Ewing, the study’s co-author and a professor of city and metropolitan planning at the University of Utah.
What was your study about and what was its goal?
What’s different about this particular study is that it’s the first meta-analysis.
Since about 1990, [there have been] lots of individual studies of the effect the built environment has on travel: vehicle miles traveled, walking, transit use, number of vehicle trips. There have been roughly a dozen literature reviews, but no one has until now pulled together all those studies and produced measures of effect size.
Planners for the last 40 years have been calling for mixed-use development, but we haven’t known how beneficial mixed-use development was in terms of discouraging driving and encouraging walking and transit use. The study provides an estimate [of] just how important it is to balance jobs and housing in a neighborhood, rather than having all housing or all jobs.
The study made an attempt to generalize across something like 60 individual studies. It provides measures that can be used by planners and by policy makers to evaluate development proposals, to do health impact assessments, to do climate action plans.
If California metropolitan areas, under their smart growth climate law, double the density of their regions over the next 20 years, what effect will that have on total vehicle miles and total emissions? Now there’s a convenient way of summarizing the effects. We can say with some certainty that if you double density you’ll get a five to 10 percent reduction in vehicle miles traveled.
What did you find was the best way to minimize vehicle travel?
The best way to minimize driving appears to be to develop in existing centers near the core of the metropolitan area, in areas of high destination accessibility where there are a whole lot of jobs near by. That’s the most important single factor.
We found other factors like mixed-use and intersections and block size. They fall into a second group that is less important than destination accessibility, but are more important than density. Density turns out as less important than land-use mix where shops and schools and workplaces are near to people’s homes.
If you’re trying to minimize vehicle miles traveled and maximize walking and transit, you’re better off emphasizing mixed-use and destination accessibility than just bumping up density. A dense development in the suburbs, far from transit and employment centers and stores, is probably not going to buy you much in the way of walking and transit use. Almost any development in the central city is going to be more efficient from a transportation standpoint.
What is high destination accessibility?
It means within easy driving distance of a lot of what are referred to as “trip attractions” — shopping, employment, recreational facilities. These trip attractions are often measured by the number of jobs that you can reach by car in a given travel time. That’s the most important single variable. It’s correlated with the distance from the central business district. Generally the further out you go, the lower the destination accessibility.
What should planners and policy makers be doing differently?
What we’ve been doing since World War II is promoting and encouraging urban sprawl. Urban sprawl has poor destination accessibility, low density, minimal diversity, poor design and great distance from transit.
That’s what we’ve approved and actually encouraged by subsidizing sprawl. We should be promoting just the opposite of sprawl, which is dense, diverse, with high destination accessibility due to location within the region and with ready access to transit. [But] everything kind of works against compact development.
Compact development is somewhat more expensive to do and it’s somewhat harder to do. Lenders are not as comfortable with mixed-use development. Traffic engineers tend to encourage development on the periphery on the urban area because that’s where there’s excess road capacity.
The importance of the study is that it will assist planners by allowing them to quantify the benefits of compact development.
Do you have anything else to add?
A pretty consistent picture has emerged: that sprawl is the enemy. We’re going to have to do something about it and there are lots of reform efforts occurring around the country. What we really need is something like the climate and energy bill that kind of institutionalizes compact development and encourages it across the spectrum. Right now we have a few states with growth management and a lot of states that don’t have it. What we probably need is some federal intervention.
Photo: Reid Ewing