"Concerns arise at locations where the posted speed limit based on an 85th percentile speed exceeds the roadway's inferred design speed. This inconsistency is a result of the fact that criteria used in highway design incorporate a significant factor of safety--that is, roadways are designed for near worst-case conditions." "Concerns arise at locations where the posted speed limit based on an 85th percentile speed exceeds the roadway's inferred design speed. This inconsistency is a result of the fact that criteria used in highway design incorporate a significant factor of safety--that is, roadways are designed for near worst-case conditions."

Further Reading:

Design Speed, Operating Speed, and Posted Speed Practices