The AASHTO Roadside Design Guide defines clear zone as the "total roadside boarder 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 are."
The desired width is dependent upon the traffic volumes and speeds and roadside geometry. As an example, a roadway with 400 vehicles per day, a posted speed limit of 55 mph and 4:1 (H:V) side slopes would have a calculated clear zone of 23 feet, per figure 3.1b; AASHTO Roadside Design Guide.
The Road Design Guide cautions, the distances obtained from figure 3.1b and associated table suggests only the approximate center of a range to be considered and not a precise distance to be held as absolute.