Counterpoint | Some further context to understand Minnesota’s speed limits

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As a retired traffic engineer, I read with interest the Associated Press speed limit article published in the Minnesota Star Tribune Aug. 18. And after reading some of the letters subsequently published in the Star Tribune regarding this article, I felt the need to provide comment of my own.

First, it would be useful to provide some background about myself. I was a professional traffic engineer from 1975 to 2017, and my career included stints as the traffic engineer for Schenectady County, N.Y., from 1977 to 1981; Brooklyn Park, Minn., from 1981 to 1996, and Rochester, Minn., from 2012 to 2017. As a county and city traffic engineer, I had to deal with speed limits and safety constantly, and I conducted several studies on speeding, speed limits and safety. I also did considerable research on what others had found regarding these subjects.

The speed limit article in the Star Tribune gives the impression that the 85% rule — the speed at which that percentage of drivers travel or below — was (and still is) used to set most speed limits on roadways across the U.S. and that this rule was being used because it provides the roadway with the safest speed. Based on my experience and research, both of these impressions are misleading and not entirely true.

For example, when I was working for Brooklyn Park, the majority of city jurisdiction streets had a 30 mph speed limit. That was not the result of implementation of the 85% rule but because the Minnesota law indicated that the maximum speed limit on streets that met the criteria for an urban district street (an urban district street had to have essentially full development along the street) was 30 mph unless otherwise posted. Note that the 30 mph speed limit on these urban district streets was police-enforceable without any signage being in place. Also note that this 30 mph urban district street speed limit was applicable statewide.

There were a few city jurisdiction streets in Brooklyn Park that had a posted speed limit higher than 30 mph, mostly posted at speed limits of 35 mph or 40 mph. These streets were generally arterial or collector streets, and their speed limits were established by the state generally using the 85% rule. During my tenure as traffic engineer, most of these streets had their speed limit reduced to 30 mph because the law allowed the city to send a resolution to the state declaring that the street met the urban district street criteria and the city intended to post the street for a 30 mph speed limit. In theory, the state could challenge this and keep the existing speed limit in place. However, during my tenure, the state never challenged the city’s action.

The speed limit article gives the impression that the 85% rule is used to establish speed limits because it provides the roadway with the “safest” speed limit. Considering speed limits and safety, would not the “safest” speed limit be a low speed limit, such as 15 mph or 10 mph? That, however, would not be followed by most motorists. Most motorists are trying to get somewhere relatively quickly, and most set their speed according to their perception of the environment along the road, regardless of what may be posted on some sign along the way. The 85% rule provides the roadway with a speed limit that the majority (85%) of motorists are voluntarily willing to comply with. The 85% speed limit also minimizes the range of variation of speed of traffic. The range of variation of speed of traffic is a greater indicator of likely safety problems than the actual speed limit; the greater the variation of speed of traffic, the higher the likelihood of accidents on the road.

The speed limit article cites a study that indicated increases in the speed limits on freeways resulted in increases in the chance of fatalities. Anyone with a familiarity of the physics involved in vehicle crashes could have told you this. Increasing the speed of a vehicle will increase the chances for fatalities/serious injuries if a crash occurs. However, there was no mention in the speed limit article that freeways/interstate highways generally have the lowest crash rates compared with any other type of roadway. The roadways with the highest crash rates are the ones that have high traffic volumes, high numbers of at-grade intersections (streets and driveways) and high volumes of cross-street traffic.