The Minnesota Department of Transportation is increasing speeds on 5,240 miles of state highways based on the recommendations of a five-year study released this week. The speeds will increase from 55 to 60 miles per hour.
Of the 7,000 miles studied, speed limits ultimately will be increased on 77% of rural, two-lane state highways, according to the final report. New speed limits go into effect once new speed limit signs are posted. Most of the signs posting the new speed limits are in place, with the rest expected to be up by spring 2019.
The Minnesota Legislature in 2014 mandated that MnDOT study all Minnesota two-lane roadways with a speed of 55 miles per hour.
It is the most comprehensive study the agency has made in terms of miles studied and level of detail, according to Nathan Drews, engineering specialist in the Office of Traffic Engineering.
The study is also the largest system-wide change in Minnesota speed limits since the national maximum speed limit of 55 mph was included in President Nixon’s Emergency Highway Conservation Act bill in 1974. The Minnesota Commissioner of Highways later that year established an executive order about speed limits.
The $1.2 million study included collecting travel speed samples on each section of roadway and evaluating roadway geometrics and hazards to determine if a speed limit could be changed without affecting motorist safety.
The recommendation for a speed increase along each of these roadways considered the speed that 85% of motorists drive at or below along with an evaluation of other factors, such as access points, shoulder width, vertical grades and crash history.
MnDOT has conducted before and after studies on many roadways that recently increased to 60 mph. There was no change in the overall 85th percentile speed from before the speed limit change to after. The mean speed, which is the average speed of all drivers, increased by one mile per hour and the standard deviation, which is the measure of how spread out the drivers’ speeds were, reduced slightly.
“This means that after speed limits increased, travel speeds at the locations sampled were slightly more consistent between each vehicle,” said Drews. “In other words, more drivers traveled at a similar speed after speed limits increased. This is a desirable outcome, but this change is very slight and may not affect the frequency or severity of crashes.”
This most recent study echoes results from the previous studies. From 2006 to 2013, MnDOT increased speeds to 60 mph on 1,550 miles of two-lane rural highways. Studies conducted to determine the impact of raising speed limits on those roadways found that the overall 85th percentile speed before and after the changes were the same, the average speed increased slightly and the variation of the speeds decreased.
Drews said a properly selected speed limit can potentially increase the safety of the roadway by creating uniform travel speeds for all vehicles.
MnDOT plans to study the effect of the changes over the next several years to ensure these roadways continue to operate safely.