An important part of a traffic investigation is the speed study. Drivers take into consideration the conditions of the roadway environment when determining their comfortable driving speed. Thus, the speed which the majority of the drivers consider prudent is an important factor.
Roadway data is collected using automated traffic counters at select locations during idea driving conditions. A technical analysis is done to determine the 85th percentile. This 85th percentile represents the speed of which people drive during idea conditions. Experience has shown that a posted speed limit near this value is the maximum safe and reasonable speed most drivers will travel.