The 85th percentile speeds may differ considerably by direction at some locations. Such conditions are usually caused by relatively heavy development on one side of the road. Next to the development, motorists will tend to drive slower because of interference from traffic to and from the development.

On divided highways

, the zone speeds should conform to the 85th percentile speed even though this may require zoning for different speeds in opposite directions.

On undivided roadways

, the zones in opposite directions should be the same for enforcement purposes.