The objectives of this study were to examine current criteria and procedures used for setting speed limits on public roads and to recommend appropriate maximum speed limits for various types of roads. The study involved three major areas. These were: review of literature, collection and analysis of speed data, and collection and analysis of accident data. The speed data included collecting data on various highway types using the “moving” radar mode, use of data from speed monitoring stations, comparison of spot speed data before and after speed limit changes, and both moving and spot speed data at construction zones. Accident data were collected at locations where speed limits were changed and also on sections of adjacent interstates with different speed limits.
A recommendation is that the 85th percentile speed should be used as the standard method to establish speed limits. The speed limit should be posted in 5 mph increments and should be obtained by rounding the 85th percentile speed down to the nearest speed value in miles per hour that end in 5 or 0. The posted speed limit may be set at a speed where the minimum design speed for an entire road may allow the design speed for a specific location to be less than the speed limit. In those instances, advisory speed signs may be used to warn drivers to reduce their speed to less than the posted speed limit. An exception to the use of the 85th percentile speed would be for legislatively mandated speed limits.
The speed data show that a large percentage of vehicle speeds exceed currently posted speed limits. The highest percentage exceeding the limit was on urban interstates and two-lane parkways where the speed limit was 55 mph. The speeds for trucks were slightly less that for cars on all types of highways with a difference of less than 5 mph when the 85th percentile speed is considered.
A comparison of speed data at locations where speed limits were changed showed differences but not dramatic changes. The data support the theory that drivers will drive what they consider an appropriate speed regardless of the speed limit. A comparison of accident rates at adjacent sections of interstate where the speed limit was 55 and 65 mph did not find a substantial difference in the total, injury, or fatal accident rates.
Maximum speed limits are given for various type s of roads. In many instances, different limits are given for cars and trucks. The speed limit for a specific location would be based on an engineering study which would consider such factors as operating speed, design speed, roadway design elements, roadside appurtenances and obstacles, operational features, and an accident analysis. Statewide maximum speed limits of 70 mph for cars and 65 mph for trucks are recommended as well as 35 mph in business or residential areas.
Digital Object Identifier
Agent, Kenneth R.; Pigman, Jerry G.; and Weber, Joel M., "Evaluation of Speed Limits in Kentucky (KYSPR 96-172)" (1997). Kentucky Transportation Center Research Report. 323.