It makes intuitive sense that people living or working in walkable, in-town locations would not need to drive as often, or as far, as those in areas where things are farther apart. Some destinations - the dry cleaner's, maybe, or a restaurant - may be within walking distance and not require a vehicle trip at all; others, such as a downtown job, may be accessible by transit service that is more frequent and convenient than typically found in spread-out locations; and trips that are necessarily or more conveniently taken by car may not require trips that are lengthy.
The result, in professional transportation lingo, is that development in more urban locations is unlikely to "generate" as much motor vehicle travel as does development in suburban and rural locations. And we don't have to rely on intuition alone to know this. In a massive "meta-analysis" (a study of studies) that I still consider to be the definitive study of how land use affects travel behavior, Professors Reid Ewing (University of Utah) and Robert Cervero (UC-Berkeley) found that the more centrally located a given residence, the fewer vehicle miles traveled from that residence. The better connected a neighborhood and the more convenient it is to walkable destinations, the more trips are taken by walking rather than by driving.
These findings are consistent with a wealth of transportation research finding, for example, that transit corridors reduce traffic and increase walking; that development close to transit generates far fewer driving trips than would otherwise be expected; and that development consistent with the location and design standards of the green rating system LEED for Neighborhood Development reduces driving.
This is all critically important to the environment, because fewer and shorter driving trips lead to reduced emissions of carbon dioxide and other vehicle pollutants. It is also important to the economy, because lower rates of vehicle use require the construction and maintenance of less infrastructure to accommodate driving. (And, apart from reducing traffic, infill doesn't consume undeveloped land.)
So far, so good for those who advocate "smart growth" and close-in living. There is a fly in the ointment, though: the expectations of neighbors living near proposed development and, far too often, municipalities that must issue development approvals and construction permits are generally based on the assumption that these driving reductions associated with urban infill and transit-served development do not exist.
Indeed, the problem is institutionalized. Most municipalities, when estimating the amount of traffic that a new development will generate, rely on standard predictions found in one of the bibles of the transportation industry, the Trip Generation Manual of the Institute of Transportation Engineers (ITE). A nice quick summation of the manual was recently furnished by a group of academic researchers, writing in the University of California publication Access:
"This manual provides automobile trip generation rates--the number of cars and trucks expected to enter and exit a site per hour--for land uses ranging from apartments and offices to coffee shops and bowling alleys . . . Local governments use them to evaluate developments in vastly different geographic areas, from the suburban fringe to the city center. The rates are typically derived using the simple relationship between the size of a development and the predicted number of trips generated by that development during a particular time of day (e.g., the number of trips per 1,000 gross square feet of retail, during the afternoon peak hour).
"ITE estimates trip generation rates using data collected by practitioners and researchers at development sites throughout the United States. To control for external influences on automobile trip generation rates, ITE requires data be collected at isolated, single-use sites that are neither served by transit nor typically accessed on foot or by bicycle."
Note that last sentence. By design, the standard forecasts used throughout the transportation industry do not consider the kinds of walkable, mixed-use environments found often in city neighborhoods. As a result, the forecasts typically overestimate the amount of traffic likely to be associated with much urban development. And that's a problem for environmentalists and other proponents of urban development alternatives to suburban sprawl.
How far off-target are the ITE predictions when applied to walkable infill development? A lot, according to three researchers who have examined the data in depth. In the Access article referenced above and just published, Robert Schneider (University of Wisconsin-Milwaukee), Susan Handy (UC-Davis), and Kevan Shafizadeh (Cal State-Sacramento) write that, in their analysis, "ITE rates overestimated vehicle trips by an average of 2.3 times in the morning peak-hour period and by 2.4 times in the evening peak-hour period." In other words, smart growth projects generated, on average, less than half the amount of traffic that standard engineering estimates had predicted.
The researchers collected empirical data from thirty land uses in central areas of Los Angeles, Oakland, Sacramento, and San Francisco. For office buildings, the overestimates were even larger (ITE rates were 2.9 times greater in the morning peak hour and 3.2 times greater in the evening); for high-density, residential-only buildings, the ITE estimates were closer but still too high (1.1 times greater in the morning, 1.4 times greater in the evening).
The real excitement in this research, however, comes from development situations where a number of factors influencing driving rates are present. Ewing and Cervero's meta-analysis, for example, examined the influence of not just centrality of location but also the street network and design, the diversity of walkable land uses, the neighborhood density, and the presence or absence of transit. They found the effects of various factors to be additive when two or more were present. How might we more accurately predict traffic generation from, say, an apartment building with ground-floor retail, one mile from the downtown core and next to a rail transit stop with frequent service?
To that end, Schneider, Handy and Shafizadeh have developed a new "smart growth trip generation tool" that attempts to provide quantitative adjustments to the standard ITE predictions based on whether, and which, smart growth factors are present. They attempt to measure not just likely car trips but also those that will be taken on foot, by transit, or by bicycle. Their hope is generally to improve the accuracy of traffic forecasting and more specifically to influence the next-generation ITE Trip Generation Manual. The tool is described in more detail in their article.
Let's wish them luck. People's reactions to proposed new development should be based on best-available data, not worst-case fears.
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Kaid Benfield writes about community, development, and the environment on Huffington Post and in the national media. Kaid's latest book, People Habitat: 25 Ways to Think About Greener, Healthier Cities, is available from booksellers nationwide.