"Road diets" are often conversions of four-lane undivided roads into three lanes (two through lanes plus a center turn lane) (Figure 1). The fourth lane may be converted to bicycle lanes, sidewalks, and/or on-street parking. In other words, the existing cross-section is re-allocated. Road diets can potentially offer benefits to both vehicles and pedestrians. On a four-lane street, drivers change lanes to pass slower vehicles (e.g., vehicles stopped in the left lane waiting to make a left turn). In contrast, on a road diet, driver speeds in each through lane are limited by the speed of the lead vehicle. Thus, road diets may reduce vehicle speeds and vehicle interactions during lane changes, which could potentially reduce the number and severity of vehicle-to-vehicle crashes. Road diets may also benefit pedestrians, because they will have fewer lanes of traffic to cross and motor vehicle speeds are likely to be lower. Zegeer et al. found a reduction in pedestrian crash risk when crossing two- and three-lane roads, compared to roads with four or more lanes. Under most average daily traffic (ADT) conditions tested, road diets have minimal effects on vehicle capacity, because left-turning vehicles are moved into a common two-way left-turn lane. However, for road diets with ADT's above approximately 20,000 vehicles, there is an increased likelihood that traffic congestion will increase to the point of diverting traffic to alternate routes. Simulation analyses by Knapp and Giese indicated that conversions from four to three lanes would reduce level of service when peak-hour volumes exceed 1,750 vehicles per hour. While these potential crash-related benefits are cited by road-diet advocates, there has been only limited research concerning such benefits. (States and cities where the effects of road diets on crashes have been evaluated include Iowa, San Francisco, and Seattle.) This study was designed to help fill this gap.


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Media Info

  • Features: Figures; References;
  • Pagination: 10p

Subject/Index Terms

Filing Info

  • Accession Number: 00932464
  • Record Type: Publication
  • ISBN: 0935403671
  • Report/Paper Numbers: CD-016
  • Files: TRIS
  • Created Date: Oct 24 2002 12:00AM