The 1990 AASHTO's "A Policy on Geometric Design of Highways and Streets" contains information on three superelevation design procedures: rural highways and high-speed urban streets, low-speed urban streets, and curvature of turning roadways and curvature at intersections. This report reviews the history of the horizontal curve design procedures through the published policies (1940 to 1990), presents findings from the literature on key issues, and discusses additional research needs on side friction factors and transition length determination. Side friction factors used in high-speed and low-speed design were determined using vehicle occupant comfort as the selection criterion. This criterion assumes that drivers limit their speed on curves to ensure comfort, and that discomfort is directly related to the unbalanced side-friction. Several concerns or issues accompany these assumptions. For example, the speed at which discomfort (or side pitch) first becomes noticeable may be slower than necessary for comfort or safety and the level of discomfort felt by a driver may not be solely related to side friction only. These assumptions also do not directly consider vehicle characteristics or constant safety factors over the range of design speeds. Transition length determination for high-speed and intersection design is based on appearance and comfort. The criterion was developed to avoid an appearance that results from too rapid a change in superelevation. For low-speed design, a change in acceleration over the change in time factor, known as C, is used to determine superelevation runoff. High-speed design includes factors that are to be used to determine runoff lengths for roads with more than two lanes. Low-speed design does not include similar factors to adjust for wider pavements; however, it does include a method for adjusting runoff length for radii larger than minimum that the high-speed design procedure does not include.

  • Supplemental Notes:
    • Research study title: Material Development -- Superelevation Section -- TxDOT Design Manual.
  • Corporate Authors:

    Texas Transportation Institute

    Texas A&M University System, 1600 E Lamar Boulevard
    Arlington, TX  United States  76011

    Texas Department of Transportation

    Transportation Planning Division, P.O. Box 5051
    Austin, TX  United States  78763
  • Authors:
    • Fitzpatrick, K
    • Kahl, K
  • Publication Date: 1992-9


  • English

Media Info

  • Features: Figures; References; Tables;
  • Pagination: 82 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00628810
  • Record Type: Publication
  • Report/Paper Numbers: TX-92/1949-1, Res Rept 1949-1, TTI: 2-8-92-1949
  • Contract Numbers: Study 2-8-92-1949
  • Created Date: Apr 14 1993 12:00AM