Urban and rural trunkline intersections are examined with regard to their wet accident percentages. The examination first takes account of the estimated percentage of surface wet time for each month from January to December. Because precipitation data are only available for the amount of precipitation for designated time intervals, a method is developed to convert these data into percent wet time--a factor necessary in assessing wet surface exposure. Using this conversion method, the precipitation data from 120 weather stations are transformed to give a month by month wetness profile for the entire state for the years 1963 to 1974. The range in monthly wetness for this period is from less than 1% to more than 25%. This potential 25 to 1 ratio is very influential in wet accident incidence and should be taken into account before other wet accident variables are examined. Nearly 40,000 accidents occurring at over 2,000 intersections for which a skid number value was available were tabulated to provide wet accident proportions. These data together with the location's wet time proportion, as estimated from the nearest weather station, provided an opportunity to statistically fit the wet accident model for the variables included, and for each of three surface types in common use. The fit is satisfactory and suggests an accelerating function for skid number; for all levels of wetness, and taking all surfaces together, a skid number less than about 30 is accompanied by an accelerating increase in wet accident percentages; although the actual shape of the curve depends on wet time. In general, it appears that there is no skid number hazard threshold--wet accident incidence increases monotonically and continuously, albeit at an increasing rate, as skid number deteriorates. This relationship appears strongest for the bituminous aggregate surfaces.

  • Supplemental Notes:
    • Conducted in cooperation with DOT, Federal Highway Administration.
  • Corporate Authors:

    Michigan Department of State Highways & Transport

    State Highways Building, P.O. Drawer K
    Lansing, MI  United States  48904
  • Authors:
    • Holbrook, L F
  • Publication Date: 1976-10

Media Info

  • Pagination: 57 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00158149
  • Record Type: Publication
  • Source Agency: Federal Highway Administration
  • Report/Paper Numbers: FHWA-MI-76-994 Final Rpt.
  • Contract Numbers: HP&R
  • Created Date: Sep 28 1977 12:00AM