What are the advantages and disadvantages of a road diet?

The road diet is a relatively new concept. Many roadways have been overbuilt to keep pace with increases in automobile travel but some roadways actually perform worse with the additional lanes. A road diet solves this problem by removing unneeded lanes or narrowing existing travel lanes to reallocate space for other needs (pedestrian paths, bicycle lanes, transit facilities, etc.).

Burden and Lagerwey (1999) examine the impact of both four-to-three and four-to-two lane conversions (four-to-three removes two travel lanes and adds a shared left turn lane). The article identifies streets with four or more lanes and less than 15,000 ADT as potential candidates for a road diet. Road diets have been implemented in cities such as Seattle, Washington; Hartford, Connecticut; and Toronto, Ontario.

Successful road diets have been achieved on streets carrying as much as 20,000 ADT, however lane removal on higher volume roadways should be studied carefully to ensure that traffic controls and access management are appropriate for larger volumes of traffic.

In 2006 the Highway Safety Information System issued "Evaluation of Lane Reduction 'Road Diet' Measures and Their Effects on Crashes and Injuries." This report on the performance of road diets in California and Washington found crash rates to be six percent lower on streets with road diets compared to similar streets without treatments.

Perceived disadvantages of a road diet may include increased travel times. Increasingly, the "time saved" argument does not convince adjoining landowners and local residents asked to support the widening of a road. At public hearings, the benefit of reduced travel time is rejected in favor of improved quality of life and lower speeds caused in part by reasonable traffic congestion.

References and resources

Burden, D. and Lagerwey, P. (1999). Road diets: fixing the big roads. http://www.walkable.org/assets/downloads/roaddiets.pdf

Gates, T. J., Noyce, D.A., Talada, V., & Hill, L. (2007). The safety and operational effects of "road diet" conversions in Minnesota. Washington, D.C.: Transportation Research Board. http://pubsindex.trb.org/document/view/default.asp?lbid=801948

Highway Safety Information System. Summary report: Evaluation of lane reduction "road diet" measures and their effects on crashes and injuries. HSIS FHWA-HRT-04-082. http://www.tfhrc.gov/safety/hsis/pubs/04082/index.htm

Noland, R.B. (2002). Traffic fatalities and injuries: The effect of changes in infrastructure and other trends. http://www.cts.cv.ic.ac.uk/staff/wp22-noland.pdf

Zegeer, C.V., Stewart, J.R., Huang, H.F., and Lagerwey, P. (2001). Safety Effects of Marked vs. Unmarked Crosswalks at Uncontrolled Locations: Executive Summary and Recommended Guidelines, FHWA-RD-01-075. McLean, Va.: U.S. Department of Transportation.