Crash Experience

·        Preliminary evaluation – Collect crash history for the most current three year period (or longer) along the route segment.  Look at all single vehicle crashes that are not at intersections and are not intersection related, occurring on the shoulder or off the road, both to the left and to the right.

1.       Determine the average number of crashes per year on the segment.  Divide the annual number of crashes by the segment length, in miles.  Compare that number against 0.6 run-off-the-road crashes per mile for rural undivided highways with shoulder widths of 6 feet or greater and speeds of 45 mph or greater.

2.       Determine the current average annual traffic volume (AADT*365) and multiply by the section length to determine the annual miles of vehicle travel for the segment.  Divide the annual miles of vehicle travel by one hundred million.  Divide the average number of crashes per year on the segment by the annual vehicle miles traveled (expressed in units of one hundred million).  This will yield the annual number of crashes per one hundred million miles of vehicle travel.  Compare that number against a statewide average of 34 run-off-the-road crashes per one hundred million miles of vehicle travel for rural undivided highways with shoulder widths of 6 feet or greater and speeds of 45 mph or greater.

These calculations will give you a general idea of whether your project is experiencing a high number of run-off-the-road crashes, but this preliminary evaluation process does not yield conclusive evidence regarding the need for shoulder rumble strips.  When the run-off-the-road crash experience on your project approaches or exceeds either of the statewide average values, it is an indication that your project warrants additional evaluation for shoulder rumble strips.  Routes with higher volumes should give a little more weight to the crashes per one hundred million miles of vehicle travel value than to the crash per mile value.  

Contact Information

Dave Olson, Design Policy, Standards, and Research Manager
Design Policy & Standards @wsdot.wa.gov
360-705-7952  FAX: 360-705-6815
Washington State Department of Transportation Design Office
PO Box 47329
Olympia, WA 98504-7329

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