Speed zone studies
Speed limits on Texas highways are set by the 85th percentile method, which represents the speed the majority of drivers will be traveling at or below. This is a sound engineering principle used to set speed limits on highways nationwide for the past 60 years.
Speed checks are conducted to determine the 85th percentile speed. The observed free-flowing speed for vehicles is tallied and the 85th percentile speed is calculated using gathered information. To ensure a true reflection of a normal traffic situation, speed checks are made on average weekdays during off-peak hours, under favorable weather conditions.
The data collected typically shows roadway features such as curves, surface width and type, right-of-way width, crash history, cross streets, school crossings and sites that generate traffic. Engineers design speed zones based on this information.
The speed limit is normally set at the nearest value to the 85th percentile speed ending in 5 or 0. The posted limit may be lowered up to 10 miles per hour (12 miles per hour for locations with crash rates higher than statewide average) below the 85th percentile speed if some of the following conditions are present:
- Roadway pavement width of 20 feet or less
- Curves and hills
- Hidden driveways and other developments
- High number of driveways
- Crash history
- Rural residential or developed area
- Lack of striped, improved shoulders
Once the study is completed, black on white speed limit signs are posted along the roadway to alert drivers of the maximum legal speed for that section of roadway.
Can a speed limit be too low?
There are disadvantages to setting speed limits far below the 85th percentile speed. If reasonable drivers see an unreasonably low speed limit without seeing a need to drive that slowly, they tend to ignore the signs and develop disrespect for speed limits in general.
When a speed limit is set below the 85th percentile, law enforcement officials must deal with reasonable people being ticketed for exceeding the posted limit as well as motorists who drive too fast.