What is the primary factor utilized to determine speed limits?

The primary factor utilized to determine speed limits is the 85th percentile speed. This concept is based on traffic engineering principles, which assert that the speed at or below which 85% of drivers travel under free-flowing conditions is a reasonable indicator of the safe and appropriate speed for that road segment. By setting the speed limit at this percentile, authorities aim to establish a limit that reflects the natural behavior of drivers, ensuring it is both safe and enforceable. This method acknowledges that most drivers will comply with the limits if they are aligned with their driving habits, enhancing the overall efficiency of traffic flow and safety on the roads.

Other options may suggest methods of determining speed limits, but they do not focus on the empirical and data-driven approach that the 85th percentile offers. For example, the average speed of all drivers can over-represent slower drivers and under-represent faster drivers, while the speed of the fastest vehicle could encourage unsafe driving behavior. Manufacturer recommendations are not usually relevant since they pertain to vehicle performance rather than roadway safety considerations.