Chapter 5 - Roadside Safety and Horizontal Clearance

5.2. Rural Shoulder Horizontal Clearance

Horizontal clearance is the lateral distance measured either from the traveled way or the face of curb, to the face of a roadside object or feature. The rural shoulder is the part of the roadway beyond the edge of traveled way that is graded or paved flush with the edge of traveled way to allow for emergency usage (AASHTO 2006).

Horizontal clearance for rural type shoulders, including graded or paved surfaces, is based on the concept of clear zone that is established by the AASHTO Roadside Design Guide . By definition, clear zone is the area beyond the roadway edge of traveled way which provides an environment free of fixed objects, with stable, flattened slopes which enhance the opportunity for reducing crash severity (AASHTO, 2006).  Fixed objects include trees, large shrubs, bodies of water, and elements of the roadway facility such as road signs, structure piers, utility poles or light standards, and electrical or controller cabinets, or other non-moveable objects that can pose a safety hazard to a vehicle and its occupants if the vehicle leaves the roadway.

In determining the acceptable clear zone for a particular roadway and prevailing condition, refer to the current AASHTO Roadside Design Guide in its entirety, and not just to the tables provided in Chapter 3 of the Guide.  Principles of clear zone safe drainage treatments, ditch designs, curve correction factors, and many other features of the clear zone concept are key elements to an overall safe and aesthetically pleasing roadside design. It is not the intent of this Manual to reproduce the clear zone values that are provided in the AASHTORoadside Design Guide.

It is desirable to provide the maximum clear zone that is commensurate and practical for the prevailing conditions. The maximum clear zone values, based on the traffic volume, slope, geometry and speeds identified in the current AASHTORoadside Design Guide should be utilized when providing full reconstruction of the roadway. If not practical to provide the recommended upper value due to overall highway design considerations, the minimum values should be observed, for the respective conditions. For retro-fit types of projects, achieving the minimum clear zone values are acceptable.

The minimum horizontal clearance for the lowest type of rural roadways (local and collectors) is 10-ft., regardless of traffic volumes.

Features or objects located within the accepted clear zone for a roadside should comply with the guidelines provided in the AASHTO Roadside Design Guide. If features or fixed objects cannot be removed or modified to become clear zone compliant, they should be shielded in a cost effective manner that is consistent with current practice and standards.  It is GDOT’s policy that fixed objects in median areas of 64-ft. or less that can not be eliminated shall be treated with cost-effective shielding devices, such as guardrail, impact attenuators, or earth-mound redirection design. 

In cases where road median widths are greater than 64-ft., but less than 84-ft., specific engineering judgment should be made by the designer.  For medians wider than 84-ft., it is not necessary to protect fixed objects that are located near the center of the median and outside the required clear zone.  For roadsides, it is GDOT’s policy to shield objects that are within the defined clear zone. The intent of the designer should be to reduce the seriousness of the consequences of a vehicle leaving the roadway.