What is the primary use of the 85th percentile speed?

• A. Determining the average speed of traffic
• B. Evaluating post speed limits
• C. Calculating the K factor
• D. Identifying traffic volume

Answer: (B)

The primary use of the 85th percentile speed is evaluating posted speed limits. This method is employed to establish a speed limit that reflects the natural flow of traffic on a particular roadway segment. By determining the speed at which 85% of drivers operate below, transportation agencies can set a speed limit that is reasonable and compliant with the driving behavior observed on that segment of road. This practice is rooted in the understanding that most drivers tend to drive at a speed that they perceive as safe and appropriate for the conditions, and aligning the posted limit with this behavior enhances compliance and safety.

Additionally, using the 85th percentile speed helps in promoting effective traffic management and improving overall road safety by reducing the number of speed limit violations. The goal is to create a speed limit that encourages adherence and respects the flow of traffic without being arbitrarily rigid. Hence, employing the 85th percentile speed is crucial for ensuring that speed limits are set based on empirical data, leading to a better driving environment.