Speed limits on all roadways should be set based on spot speed studies and the 85th percentile operating speed (see Chapter 3, , of this manual). Legal minimum and maximum speeds should establish the boundaries of the speed limits. If an existing roadway section’s posted speed limit is to be raised, the roadway’s roadside features should be examined to determine if modifications may be necessary to maintain roadside safety.

It is appropriate for posted speed limits to be based on the 85th percentile speed, even for those sections of roadway that have an inferred design speed lower than the 85th percentile speed. Posting a roadway’s speed limit based on its 85th percentile speed is considered good and typical engineering practice. This practice remains valid, even where the inferred design speed is lower than the resulting posted speed limit. In such situations, the posted speed limit would not be considered excessive or unsafe.

Arbitrarily setting lower speed limits at point locations due to a perceived shorter than desirable stopping sight distance is neither effective nor good engineering practice.

If a section of roadway has (or is expected to have) a posted speed in excess of the roadway’s inferred design speed and a safety concern exists at the location, then appropriate warning or informational signs should be installed to warn or inform drivers of the condition. Slightly shorter than desirable stopping sight distances do not present an unsafe operating condition, because of the conservative assumptions made in establishing desirable stopping sight distances. It is important to remember that any sign is a roadside object and that it should be installed only when its need is clearly demonstrated.

New or reconstructed roadways (and roadway sections) should be designed to accommodate operating speeds consistent with the roadway’s highest anticipated posted speed limit based on the roadway’s initial or ultimate function.