Smart Growth Communities Require Less Pavement
Two recent studies provide further evidence that current planning practices require economically-excessive road and parking supply, a problem that becomes more severe as vehicle travel peaks and demand for alternative modes and more multi-modal communities increases.
Phantom Trips: Overestimating the Traffic Impacts of New Development by Professor Adam Millard-Ball critically examined the methods commonly used to predict trip generation, that is, the additional vehicle traffic resulting from new development. The study found that commonly used methods, such as the ITE Trip Generation Manual:
• Overestimate trip generation in smart growth locations.
• Use biased samples, since surveys are generally performed at “successful” sites.
• Assume that any trips made from a new building are “new” trips, rather than shifts of existing regional trips that would occur if the building had not been constructed.
As a result of these factors the author concludes that current practices greatly overestimate the number of vehicle trips that can be attributed to any development project, resulting in economically-excessive urban roadway capacity and a structural bias against infill development.
Trip Generation for Smart Growth Projects by Robert J. Schneider, Susan L. Handy and Kevan Shafizadeh applied a new, more rigorous data collection method to count vehicle trips at urban sites. The results indicate that commonly-used trip generation prediction models significantly overestimate trip generation in smart growth locations, by an average of 2.3 times at the sites studied. The results were used to develop the Smart Growth Trip-Generation Adjustment Tool which can be used to adjust available trip-generation rates for smart growth development projects. Most of these findings are transferable to parking generation analysis.
These studies provide further evidence that smart growth development can provide large savings and benefits, including savings to households (from reduced vehicle expenses), developers (from less parking and transportation impact costs) and governments (from reduced roadway costs), and that current planning practices are significantly biased in favor of sprawl and automobile-dependency. These studies provide practical guidance to help communities devote less land to vehicles and more land to people.