Allowing lower auto travel through destination accessibility

Meta-analysis of planning

Jon Boyd, Jul 02, 10.

“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?” Christina Hernandez poses this question in the blog, Pure Genius.

Hernandez interviews Reid Ewing, from the University of Utah, co-author of what he calls a “meta-analysis” of planning studies recently published in the Journal of the American Planning Association.
A key metric of sustainable communities is vehicle miles traveled (VMT). The meta-analysis generalizes the results of around 60 planning studies. For example, Ewing claims this analysis quantifies the relationship between density and VMT: doubling density yields a 5-10% reduction in VMT. Therefore, density alone is not a primary factor in VMT reduction.

Ewing says, “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.” “Destination accessibility,” or the number of trip attractions that can be accessed within a fixed time frame, is a secondary factor. The number of jobs is often used in studies as a proxy for trip attractions, but the meta-analysis suggests proximity to the central business district is correlated with destination accessibility.

Mixed-use development also reduces VMT:

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.

(Image credit: Houston Tomorrow. Image is an abstract portrayal of activity intensity of the Houston region. Activity intensity is shown by adding together the population and jobs in each 1 mile square across the region, with areas of density shown in dark brown and low density shown in green. Data derived from the Houston-Galveston Area Council’s 2035 Forecasts.)