EFFECT OF THE 65 MPH SPEED LIMIT ON TRAVEL SPEEDS AND RELATED CRASHES. FINAL REPORT

Speed data from 11 states were analyzed to determine the effects of the increased MSL on travel speed and speed variance on 65 mph rural interstates and the spillover effects on 55 mph roads adjacent to them. Differential car/truck speeds, time of day and day of week effects were also examined. On 65 mph rural interstates, average speed increased almost 4 mph, 85th percentile speed increased almost 5 mph, and speed variance increased about 12%. The percent of vehicles exceeding 70 mph increased from 6% to 19%. These increases, which are larger than any trend before the increased MSL, occurred mainly between 1986 and 1988. Since 1988, only minor increases occurred. Increases in mean speed were slightly greater during daytime hours than during nighttime hours and greater on weekends than on weekdays, effects which appeared to be related to changes in the relative truck/car volumes. While a small spillover effect was observed on rural arterials adjacent to 65 mph interstates, the 0.8 mph increase was within the range of the general upward trend on all rural arterials in the 40 states with a 65 mph MSL. Speed changes on 55 mph interstates adjacent to 65 mph interstates were also within the range of trends existing before the increased MSL. Car speeds were 3.5 mph faster than truck speeds on interstates with a uniform 65 mph MSL while car speeds were 6 mph faster than truck speeds on interstates with a differential 65/55 mph limit. Car speeds were about 3 mph faster in states with a 65 mph limit for trucks than in states with a 55 mph limit for trucks. An analysis of accidents in rural interstate segments near 28 of the speed study sites indicated that the rates of injury accidents increased between 1986 and 1988. There were too few fatalities in these segments to draw definite conclusions about fatalities. The observed changes in accident rates could not be conclusively linked to the magnitude of changes in mean speed or percent of vehicles exceeding 70 mph.

• Corporate Authors:

  Last Resource, Incorporated

  309 Armagast Road
  Bellefonte, PA  United States  16823

  National Highway Traffic Safety Administration

  1200 New Jersey Avenue, SE
  Washington, DC  United States  20590
• Authors:
  • Mace, D J
  • Heckard, R
• Publication Date: 1991-3

Media Info

• Features: Appendices; Figures; References; Tables;
• Pagination: 94 p.

Subject/Index Terms

• TRT Terms: 55 mph speed limit; 65 mph speed limit; 85th percentile speed; Automobiles; Before and after studies; Crashes; Days; Differentials (Gears); Fatalities; Injuries; Interstate highways; Night; Rural highways; Speed; Speed distribution; Speed limits; Traffic crashes; Traffic speed; Trend (Statistics); Trucks; Variance; Weekdays; Weekends
• Identifier Terms: National Maximum Speed Limit
• Uncontrolled Terms: Differential speed; Speed variance
• Subject Areas: Highways; Motor Carriers; Operations and Traffic Management; Safety and Human Factors; I73: Traffic Control; I81: Accident Statistics;

Filing Info

• Accession Number: 00622673
• Record Type: Publication
• Source Agency: National Highway Traffic Safety Administration
• Report/Paper Numbers: LRI 91-01, HS-807 764
• Contract Numbers: DTNH22-88-C-07216
• Files: HSL, TRIS, USDOT
• Created Date: Jun 30 1992 12:00AM