NMA Article: How Should Speed Limits Be Set?
By Bonnie Sesolak, NMA Development Director
ôRealistic” speed limits should invite public compliance by conforming to the behavior of normal responsible drivers. This results in the safest and most efficient use of streets, roads, and highways.
Traffic engineering research has determined that speed limits should be established according to the 85th percentile of free flowing traffic. This means the limit should be set at a speed that 85 percent of people are driving at or below.
Numerous studies have shown that the 85th percentile is the safest possible speed limit. It recognizes that most drivers voluntarily adjust their speed to the total roadway/roadside environment (width, alignment, surface condition, roadside development, pedestrian activity, weather, light conditions).
It is not appropriate to set a limit below the 85th percentile to accommodate factors that are obvious to road users. Drivers are already taking these factors into account with their existing speed decisions.
Main Factors Used to Set Speed Limits:
Speed zones are established on the basis of an engineering study. The engineering study includes an analysis of the current speed distribution of free- flowing vehicles.
85th percentile speed
The speed at or below which 85 percent of the sample of free flowing vehicles are traveling. This speed should be determined by conducting a speed survey following the procedure contained in the Manual of Transportation Engineering Studies.
Advisory speed limits
These should be implemented when certain features, such as sharp corners, require a momentary reduction of speed.
Each speed zone should be periodically restudied to determine that the established speed limit is appropriate. The suggested maximum interval is five years.
In addition, an engineering study should be conducted whenever there is a change in the roadway that would affect the prevailing speed. Such changes would include elimination of parking, added lanes, signal coordination and changes in roadside development.
According to an Institute of Transportation Engineers Study, those driving 10 mph slower than the prevailing speed are six times as likely to be involved in an accident.
For more information, studies, and articles on speed limits, visit the speed limit section of the NMA’s website.
© 2008 NMA