Roadway Design and Lane Departure

Wide forgiving roadsides with obstructions removed or protected by crash cushions.

Rumble strips to warn of lane departure.

Crash Cushion Guardrails.

Wide Shoulders – For local roads and streets, a minimum clear zone of 7 to 10 feet is considered desirable on sections without curb. In the discussion on collectors without curbs, a 10-foot minimum clear zone is recommended.

Clear Zone – The Roadside Design Guide defines a clear zone as the total roadside border area, starting at the edge of the traveled way, available for safe use by errant vehicles. This area may consist of a shoulder, a recoverable slope, a non-recoverable slope, and/or a clear run-out area. The desired minimum width is dependent upon traffic volumes and speeds and on the roadside geometry. Simply stated, it is an unobstructed, relatively flat area beyond the edge of the traveled way that allows a driver to stop safely or regain control of a vehicle that leaves the traveled way.

FHWA – Clear Zone and Horizontal Clearance


Transportation Research Board

The concept applied to lessening the severity of run-off-the-road accidents has been named “the forgiving roadside.” Obviously, the least hazardous roadside environment would contain no objects that an errant vehicle could impact, but would allow the driver to regain control and either stop or return to the travel lane without injury or damage. The American Association of State Highway and Transportation Officials (AASHTO) introduced the concept of a roadside “clear zone” that was relatively flat and free of obstructions. It has become generally accepted that the width of the clear zone should depend on speed, and should be wider at those locations (like the outside of horizontal curves) where vehicles are more likely to leave the traveled portion of the roadway.


Quote from the Highway Safety Desk Book:

“The Washington State Department of Transportation has stated, “Millions of dollars are spent each year to make highways safer and the roadside features more forgiving to errant drivers. Why, then, do we tolerate parked or abandoned vehicles to remain along our highways for extended periods of time? We have designed standards that require a ‘clear zone‘ on limited access highways. Nothing can be placed in this zone without providing protection to the motorist in the form of a guardrail, barrier, crash cushions, or breakaway supports. Yet, we allow heavy vehicles to stand a few feet or even inches from the traveled lanes”

ATA (American Trucking Association) Published a safety poster stating “Trucks are Targets on Shoulders”.

Crash Lane – Is the definition of an shoulder that is not a clear zone for errant vehicles but is obstructed with dangerous objects that are not crash compatible with errant vehicles such as parked or stopped trucks, trailers, and cars.