Roadside Clear Zone

As defined by the AASHTO Roadside Design Guide(23), the roadside clear zone is 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. This area allows a driver to stop safely or regain control of a vehicle that has left the roadway. The AASHTO Roadside Design Guide recommends a clear zone width of 30 to 32 feet for flat, level terrain adjacent to a straight section of a 60 ‑mph highway with an average daily traffic of 1516000 vehicles. For steeper slopes, AASHTO recommends a 36 to 44 foot zone. For horizontal curves, AASHTO indicates the clear zone can be increased by up to 50 percent from these figures. The roadside clear zone serves the dual purpose of increasing the visibility of the crossing and traffic control devices as well as providing a safe recovery area for an errant motorist. The clear zone should be kept free of brush; trees that are more than 4 inches in diameter or that may obscure traffic control devices; and rocks, eroded areas, standing water , or other defects that may entrap an errant vehicle or lead to deterioration of the roadway or track structure. The maintenance of the sight triangle, beyond highway and railroad ROW, presents a unique problem. Except for portions on the railroad ROW, this often involves private property. The removal of trees, brush, crops, buildings, signs, storage facilities, and other obstructions to the driver’s view requires the landowner’s permission to access to the property to remove the obstruction.