The ability of different drivers varies from skillful to those who would not be able to pass an examination for an operator’s license if required to take one. It would not be wise to post safe speeds for drivers at either of the two extremes of abilities, so the selection of speeds to be posted will be aimed at the ability and performance of the average driver.

Average driver ability, of course, is considered in the form of perception — reaction time in the calculation of critical approach speeds to intersections, crosswalks, and locations with limited sight distance and in determining the posting distance for signs.