Does Compact Development Increase or Reduce Traffic Congestion?
From years of research, we know that compact development that is dense, diverse, well-designed, etc. produces fewer vehicle miles traveled (VMT) than sprawling development. But compact development also concentrates origins and destinations. No one has yet determined, using credible urban form metrics and credible congestion data, the net effect of these countervailing forces on area-wide congestion. This project is done in partnership with Utah Department of Transportation (UDOT) and support from National Institute of Transportation and Communities (NITC). The research team for this project includes Dr. Reid Ewing, Dr. Guang Tian, and PhD student Torrey Lyons.
Using compactness/sprawl metrics developed for the National Institutes of Health, and congestion data from the Texas Transportation Institute’s (TTI’s) Urban Mobility Scorecard Annual Report database, this study seeks to determine which opposing point of view of sprawl and congestion is correct. It does so by (1) measuring compactness, congestion and control variables using the best national data available for U.S. urbanized areas and (2) relating these variables to one another using multivariate methods to determine whether compactness is positively or negatively related to congestion.
Our model (and earlier studies by the same authors) suggest that an increase in compactness reduces the amount of driving people do, but also concentrates the driving in smaller areas. The former effect is slightly larger than the latter. The relationship between compactness and congestion falls short of statistical significance at the conventional 0.05 level.
This analysis does not support the idea that sprawl acts as a “traffic safety valve,” as some have claimed. However, it also does not support the reverse idea that compact development offers a one-stop solution to congestion, as others have claimed. Developing in a more compact manner may help at the margin, but the greatest reduction in congestion appears to be achievable through expansion of surface streets and higher highway user fees.