The Safety and Operational Effects of "Road Diet" Conversions in Minnesota

Research was undertaken to explore the safety and operational effects of converting four-lane undivided roadways to three-lanes with a center two-way left turn lane (TWLTL) in Minnesota. Such conversions are commonly referred to as “road diets”. Speed and/or crash data were collected for 9 road diet sites in Minnesota. Using multiple years of before-and-after data from each site, before-and-after several statistical analyses were performed using both Empirical Bayes and Grouped Comparison procedures. Based on the results of an Empirical Bayes statistical analysis of total crashes, consistent decreases in the total crashes were observed after the road diet conversions at all seven sites for which crash data were available. The range of Empirical Bayes crash reductions between the seven sites varied from 37.3 to 54.3 percent, with an overall total crash reduction of 44.2 percent (408 crashes reduced). Crashes were also analyzed by injury status (injury vs. non-injury) and type (rear end, right angle, and left turn) using a Grouped Comparison procedure, which showed a net reduction in crashes after the conversions for non-injury and right angle crashes, with crash reductions of 45.7 and 37.0 percent, respectively. A net crash reduction was also observed for rear end and left turn crashes, although the reductions were not significant. Injury crashes showed negligible change. Reductions in the mean and 85th percentile speeds after the conversions were observed at each of the six sites for which speed data were available, with a median reduction in both the mean and 85th speeds of 2 mph. The research findings suggest that conversion of four-lane undivided to three-lane TWLTL roadways is a recommended option within a given range of average daily traffic (ADT) values if the roadway of interest is experiencing safety problems related to left-turning traffic conflicting with through vehicles, which is typical of four-lane undivided roadways with several unsignalized intersections or commercial driveways.


  • English

Media Info

  • Media Type: CD-ROM
  • Features: Figures; References; Tables;
  • Pagination: 22p
  • Monograph Title: TRB 86th Annual Meeting Compendium of Papers CD-ROM

Subject/Index Terms

Filing Info

  • Accession Number: 01044906
  • Record Type: Publication
  • Report/Paper Numbers: 07-1918
  • Files: BTRIS, TRIS, TRB
  • Created Date: Feb 8 2007 6:38PM