The responsibility for setting and enforcing speed limits rests with state and local authorities, who work with engineers to determine an appropriate limit for each road. Engineers use the 85th percentile speed as a guideline. Working under the assumption that the majority of drivers travel at reasonable speeds for the given conditions, engineers calculate the speed that 85% of drivers are traveling at or below.

However, when determining an appropriate speed limit, practitioners consider other factors. The following are concepts and evaluations that guide the determination of a speed limit.

Design speed

To prepare plans and guide decisions, engineers choose a design speed. This choice takes place before new road construction.

Operating speed

This is an evaluation of the existing speeds once construction on the road is complete. The 85th percentile comes into play when determining the operating speed.

Engineering speed study

The engineering speed study helps state, county or local governments determine a regulatory speed limit to post, which is often different from the statutory speed limit established by the state legislature and takes precedence over it. These speed studies take place on all types of roadways, from city streets to interstate highways, and consider the following factors:

  • Traffic volume
  • Bicyclist or pedestrian activity
  • Crash history
  • Pavement condition
  • Sight distances
  • Number and spacing of intersections and/or driveways

Also, the engineering speed study will consider the roadway’s setting, i.e., whether it is in a rural area surrounded by farms or woodland or whether it is urban. It will also consider the features of the roadway, such as the number of lanes, the presence of hills and whether there are any curves.

Despite what many people may think, the federal government plays no part in either setting speed limits in the first place or enforcing them. That authority belongs entirely to state and local agencies within a given geographic jurisdiction.