Traffic-Safety Division

Stealth Program
For any road situation there will be a spread of vehicle speeds. There's an average speed of vehicles and some travel faster and some slower. The 50th percentile speed is the speed which 50% of vehicles are not exceeding (equally it's the speed which 50% of vehicles are exceeding). The 85th percentile speed of vehicles passing a given point is the speed at or below which 85% of the vehicles passing the point are traveling.

This is the principle value used for establishing speed controls. This method assumes that the majority of motorists are prudent and capable of selecting safe speeds; therefore, speeds established in this manner meet the legal requirement that they be reasonable and proper.

Speed Control Research
Plenty of research has shown that the safest group of vehicles are traveling at or below the 85th to 90th percentiles. Research shows that crash risk alters with speed and this is shown by the red crash risk curve.

At the 85th to 90th percentiles we tend to find drivers with above average skill and competence, and this is why their crash risk is the lowest. Above the 90th percentile we tend to find drivers exceeding safe limits and their accident risk increases as a consequence.

Note: That the "average" driver at the 50th percentile has a greater crash risk than the 85th percentile driver. Below the 30th percentile crash risk is significantly increased and these speeds tend to be used by less skilled and competent drivers.

This leads to engineering recommendations that speed limits should normally be set at around the 85th or 90th percentile speed of traffic under good conditions. There may well be some of the very safest drivers somewhat above the 90th percentile. If you are interested in stealth results in your neighborhood, contact the Safety Division.