What two speed percentiles are considered the most important?

• A. 25th and 50th percentiles
• B. 50th and 85th percentiles
• C. 75th and 90th percentiles
• D. 10th and 20th percentiles

Answer: (B)

The 50th and 85th percentiles are considered the most important in traffic engineering and transportation analysis for several reasons. The 50th percentile, also known as the median speed, represents the speed at which half of the vehicles are traveling faster and half are traveling slower. This measure is crucial because it reflects typical driving behavior and is less affected by outliers, providing a reliable benchmark for setting speed limits and designing roadways.

The 85th percentile speed is particularly significant because it indicates the speed at which 85% of drivers are traveling at or below. This measure is often used to establish safe and reasonable speed limits. It reflects the behavior of the majority of drivers, ensuring that speed limits are set at a level that is safe for most road users while also accommodating the natural tendency of drivers to travel at their desired speeds. Using the 85th percentile helps reduce the likelihood of speed-related accidents and promotes smoother traffic flow.

By focusing on these two percentiles, transportation professionals can make informed decisions about roadway design, traffic regulations, and safety measures that cater to both the majority of road users and the need for safe traveling conditions.