One of MoDOT’s most complicated jobs is figuring out what the speed limit should be on a state highway. Actually, the process of setting the speed limit is relatively simple. The complicated part is dealing with the public assumptions and political pressures that arise once a speed limit is set. In short, there are many misconceptions about speed limits. Here are a few of them:

  • Lowering the speed limit will decrease accidents and improve safety
  • Raising the speed limit will increase accidents and decrease safety
  • Drivers will usually go five mph over the posted speed limit

Decades of research prove these assumptions wrong. Here’s what actually happens:

Drivers go the speed which they perceive as safe and reasonable. Drivers base this perception on the condition of the roadway, weather conditions and the amount of traffic. The posted speed limit is ignored by many drivers. In fact, many drivers are frustrated by speed limits that are set lower than the perceived safe and reasonable speed. The few drivers that go the speed limit, regardless of how low it’s set, help create this frustration. It creates conflict between the slow and fast drivers. It reduces the gap between vehicles and makes it harder to judge the distance and speed of approaching vehicles. Bottom line: accidents increase.

The trick to setting a speed limit is finding that speed which most drivers consider safe and reasonable. This is the speed that most drivers will go. It results in fewer accidents because there is a constant speed among vehicles. Constant speed is the key. Without it, accidents increase.

MoDOT determines what the safe and reasonable speed is by conducting a speed study. A speed study is pretty much what it sounds like: it determines how fast drivers are traveling. We then set the speed limit as near as practical to the speed at or below which 85 percent of the drivers travel. This 85th percentile speed is the safe and reasonable speed. The Federal Highway Administration reports that all states and most local agencies use the 85th percentile speed as the basis for establishing speed limits. Once the 85th percentile is determined, it’s used as a sort of benchmark. Traffic engineers may adjust the actual speed limit by taking into account the number of accidents on the road, parking, pedestrians, curves and property development.

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