Examining Impacts of Increasing Speed Limit on Speed Distribution: Case Study

In January 2013, the Texas Department of Transportation increased the speed limit on a freeway through College Station, Texas from 70 mph to 75 mph. This stretch of freeway includes a data collection facility that records the speed of every vehicle. The authors accessed this data to evaluate vehicle speed distributions before and after the speed limit change. The speed data was recorded continuously and there was over 160,000 speed points in each period. Speed data were divided into groups by type of day (weekday or weekend), light condition (daylight or dark), type of vehicle (car or truck), lane position, and volume level for analysis. It was found that mean and 85th percentile speeds increased after raising the speed limit, but by less than the 5 mph increase in the speed limit. For the entire set of data, the 85th percentile speed increased from 74.7 in the before period (November 2012) to 77.0 mph in the after period (March 2013). The results indicate that the 75 mph speed limit is a better representation of the 85th percentile speed than the 70 mph speed limit. The results also indicate that, when the speed limit on a high speed road is increased, there may not a similar magnitude of increase in the 85th percentile speed.