A Powerful Case By Law Enforcement For Safe And Realistic Speed Limits

This article originally appeared in the Spring 2011 issue of Driving Freedoms, the NMA’s quarterly member newsletter.

The Michigan State Police get it. Not only do they get it, the MSP are instrumental in advocating for it with their state legislators.

“It” is the setting of speed limits based on the 85th percentile principle, something long supported by the NMA and a wide range of traffic engineers.

Studies have established that the safest limits are those based on the speed at or below that which 85 percent of unencumbered traffic is traveling. The 85th percentile speed results statistically in the lowest speed variance among vehicles, helping to alleviate congestion and minimizing the number of collisions.

Typically statutory speed limits, many of which have been unchanged for several decades, are set in the 30 to 40 percentile range. This means that by just traveling at the speed of prevailing traffic in those areas, 60 to 70 percent of the drivers will be violating the posted speed limit.

Lt. Gary Megge is a formidable spokesperson for the MSP. He recently appeared before the Transportation Committees of the Michigan Senate and House of Representatives, driving home the importance of setting realistic speed limits, i.e., those set by following these steps:

  1. Conduct a traffic study to determine the 85th percentile speed;
  2. Analyze crash data (accident rate and type of accidents);
  3. Assess the roadside environment;
  4. Consider the roadway configuration, e.g., number of lanes, length of road;
  5. Factor in other traffic and pedestrian movement influences

Lt. Megge emphasized the need to conduct the traffic studies during ideal driving conditions while eliminating any external factors that would prevent the free flow of traffic.

He also noted that the measurement of vehicle speed during the traffic study should be done unobtrusively so that drivers don’t necessarily realize that their driving habits are being surveyed.

That last point is an interesting one because the premise behind the 85th percentile speed is that regardless of what the posted speed is (or isn’t), the benchmark speed of the prevailing traffic will be the same for a given stretch of road. In other words, the majority of drivers will travel at a rate they feel safe and comfortable.

Megge demonstrated this to the Michigan legislators through the use of several PowerPoint slides, some that we have reproduced for this article. In Figure 1, Improper Change, data from two different traffic surveys at the same location (Jolly Road at West Driveway) are presented. One survey was conducted with a posted speed limit of 55 mph, the other after the limit was reduced to 45 mph. Note that the 85th percentile speed varied only from 52 mph to 51 mph between the two tests. Despite claims to the contrary from “speed kills” advocates, drivers don’t automatically speed up when posted limits are increased.

Even more dramatically, the MSP ran traffic surveys at a similar location with a wider spread between posted speed limits, from 55 mph to 70 mph. The results? As shown in Figure 2, Speed Studies, the 85th percentile speed actually decreased from 73 mph to 72 mph when the posted limit was 15 mph higher.

Megge and the MSP acted as mythbusters in another regard. Too many people believe the old saw that slower traffic flow results in safer roadways. That folly is disproven by the facts, as illustrated in Figure 3, Crash Involvement vs. Speed.

First, notice that the crash involvement curves — one each for nighttime (rural), daytime (rural), and freeway driving — bottom out close to zero deviation from average speed, or slightly to the plus deviation side. This implies that the safest traffic flow occurs when speed variance among vehicles is near zero.

Second, note how steep the crash involvement curves become when the deviation from average speed becomes more and more negative. Slower drivers create a much higher risk of collision than drivers who are at or above the average traffic speed.

The MSP have an enlightened culture, one that values traffic safety over collecting revenue through command and control tactics such as speed traps. The MSP were instrumental in drafting Michigan’s Public Act 85, a law that requires localities to base speed limits on proven, scientific methods, like that of the 85th percentile.

When several communities were still using antiquated speed limits and were issuing speeding tickets accordingly, Megge and the MSP spoke out loudly to the media about the disregard for the lawful requirements of PA 85.

It is also noteworthy that the MSP were instrumental in encouraging a change to Michigan’s standard work zone speed limits. The current “45 Where Workers Present,” as opposed to a blanket lower limit with or without workers present, might not have been enacted without the help of the MSP.

The NMA is pleased to endorse the efforts of law enforcement agencies like the Michigan State Police, who are truly helping motorists by educating both legislators and the electorate on the virtues of rational traffic regulations and proven engineering methods.

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