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FHWA Home / Safety / Roadway Departure / Clear Zones

Clear Zones

By creating Clear Zones, roadway agencies can increase the likelihood that a roadway departure results in a safe recovery rather than a crash, and mitigate the severity of crashes that do occur.

A Clear Zone is an unobstructed, traversable roadside area that allows a driver to stop safely, or regain control of a vehicle that has left the roadway. The width of the clear zone should be based on risk (also called exposure). Key factors in assessing risk include traffic volumes, speeds, and slopes. Clear roadsides consider both fixed objects and terrain that may cause vehicles to rollover.

Horizontal ‘clearance’ must not be confused with ‘clear zone’. The minimum 18 inch horizontal clearance to objects behind curbs that is specified in the AASHTO Green Book is a minimum standard offset that allows for normal traffic operations. Because curbs do not deter errant vehicles from leaving the traveled way, the minimum horizontal clearance does not provide a clear zone sufficient to accommodate errant vehicles. The recommended clear zones in the AASHTO Roadside Design Guide (RDG) are based on the design speed of the facility and the slope of the roadside and are not affected by the presence of curbs. It is recognized that providing a clear zone as recommended in the RDG may not be practical in low speed curbed facilities because of right-of-way constraints and other realities of the built environment and a design exception is not required in these cases. However, the minimum 18 inch horizontal clearance to vertical obstructions must still be met unless a design exception is approved.

Clear Zones

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Page last modified on June 24, 2011.
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