Michigan 85th Percentile Speed

Road engineers and traffic studies have shown that the best speed limit to post on a given road is the 85th percentile speed. The following description of the 85th percentile speed is taken from the publication titled "SPEED LIMITS" produced by the Maryland Department of Transportation and Maryland State Highway Commission.

What is the 85th Percentile Speed?
The 85th percentile speed is the speed at or below which 85 percent of the motorists drive on a given road unaffected by slower traffic or poor weather. This speed indicates the speed that most motorists on the road consider safe and reasonable under ideal conditions. It is a good guideline for the appropriate speed limit for that road.

Will crashes increase if the speed limit is raised?
Not really. Research has shown that the posted speed limit has little effect on the speed at which most motorists drive. Raising the speed limit does not significantly raise the speed at which motorists drive, and lowering the limit generally does not appreciably reduce speeds. However, the more motorists learn that from their driving experiences that speed limits are set at speeds they consider safe and reasonable, the greater the chances that motorists will heed them. Speed limits significantly lower than the 85th percentile speed are ignored by many drivers and difficult to enforce.

In most instances, a speed limit based on the 85th percentile speed best reflects the expectations of the largest proportion of drivers; is found by most to be a safe and comfortable limit; facilitates speed enforcement; and offers the greatest chance of achieving some uniformity in speeds on a given road. When motorists drive at a relatively uniform speed, tailgating, lane changing, and overtaking are reduced. As a result, collisions are less likely to occur.

Those who drive much faster or slower than most of the drivers around them place themselves and others at considerable risk of a collision. When the posted limit is reasonable, enforcement can be targeted to the relatively small number of drivers who exceed the limit.

So why aren't we setting Michigan roads to the 85th percentile speed?
It is much easier to set the limit below the 85th percentile speed so you have a ready pool of motorists to ticket over and over. The fines and insurance surcharges collected from most of these motorists have nothing to do with safety and everything to do with filling the police, court, and insurance companies coffers.

If we truly want safer roads we should heed what the experts have been saying all along. Set the speed limit to the 85th percentile speed and concentrate enforcement on the truly dangerous drivers.