Evaluation of Iowa's 70mph speed limit: 2.5 year update

On July 1, 2005, the State of Iowa implemented a 70 mile per hour (mph) speed limit on most rural Interstates. This document reports on a study of the safety effect of this change. Changes in speeds, traffic volume on and off the rural Interstate system (diversion), and safety (crashes) for on- and off-system roads were studied. After the change, mean and 85th percentile speeds increased by about 2 mph on rural Interstates, but speeding was reduced (the number of drivers exceeding the speed limit by 10 mph decreased from 20 per cent to about 8 per cent). Daytime and nighttime serious crashes were studied for a period of 14 and a half years prior to the change and 2 and a half years afterwards. Simple descriptive statistics reveal increases in all crash severity categories for the 2 and a half year period following the speed limit increase when compared to the most recent comparable 2 and a half year period prior to the increase. When compared to longer term trends, the increases were less pronounced in some severity levels and types, and for a few severity levels the average crash frequencies were observed to decrease. However, fatal and other serious cross-median crashes increased by relatively larger amounts as compared to expected random variation. The study also analyzed crash frequencies grouped into six-month periods, revealing similar findings.

  • Record URL:
  • Corporate Authors:

    Iowa State University. Center for Transportation Research and Education (CTRE)

    Ames, Iowa  United States 
  • Authors:
    • Souleyrette, R R
    • Stout, T B
    • Carriquiry, A
  • Publication Date: 2009-1


  • English

Media Info

  • Pagination: 24p + appendices
  • Serial:
    • Issue Number: 06-247

Subject/Index Terms

Filing Info

  • Accession Number: 01383872
  • Record Type: Publication
  • Source Agency: ARRB
  • Files: ATRI
  • Created Date: Aug 22 2012 3:58PM