The 85th percentile can be used to determine speed limits for roads. This is based on the theory that most drivers are reasonable and do not want to get in an accident, but do want to get to their destination as quickly as possible. Therefore, a speed at which 85 percent of people drive is figured to be the highest safe speed for that road.
The links at the bottom of this article explain how to calculate the 85th percentile speed and how speed limits are set.