231.2 Clear Zones
The clear zone concept is the road design principle of providing an unobstructed, traversable area beyond the edge of the traveled way for the recovery of errant vehicles. The clear zone is located adjacent to and measured from the edge of the traveled way. The clear zone includes shoulders, bike lanes, and auxiliary lane, except those auxiliary lanes that function like through lanes (>2 miles in length). The concept was developed in the 1960s and later incorporated into the Roadside Design Guide (RDG). The clear zone distances should be location specific and based on traffic volumes, design speed, roadside slopes, existing features, reasonable driver expectations, and other roadway features. These distances are neither absolute nor precise and, in some cases, it may be reasonable to leave a fixed object or non-traversable terrain within the clear zone area. The RDG Clear Zone distances are based on limited empirical data. The distances obtained from the RDG Table 3-1 should suggest only the approximate center of a range to be considered and not a precise distance to be held as absolute. For roadways with low traffic volumes, it may not be practical to apply even the minimum values found in RDG Table 3-1. Refer to RDG Chapter 12 for additional considerations for low-volume roadways and RDG Chapter 10 for additional guidance for urban applications.
Roadway design strategies for providing clear zone are as follows, in order of preference:
- 1. Remove the obstacle
- 2. Redesign the obstacle for safe traversal
- 3. Relocate the obstacle further from the roadway (or move the roadway further from obstacle)
- 4. Reduce obstacle severity (make it breakaway)
- 5. Shield the obstacle
- 6. Delineate the obstacle when it is determined to leave the obstacle.
Clear zone concepts provide safety benefits to drivers on all types of roadway projects but are primarily applicable to new construction and/or major reconstruction projects where the roadway designer has the opportunity and economy of scale to modify design elements to minimize the proximity of roadside obstacles, high fills and non-traversable terrain. Reconstructing existing roadways to create clear zone widths suggested by RDG Table 5-2 (Barrier Guidelines for Non-Traversable Terrain and Roadside Obstacles) may not be feasible or practicable given site conditions and limited resources. Clear zone concepts may also be addressed on existing roads as part of resurfacing (1R) or resurfacing and restoration (2R) projects.
Clear zone guidelines should be used when the design speed of the roadway is 45 mph or more. When the design speed is less than 45 mph, clear zones are still beneficial, but they should be used if economically feasible or identified through a safety analysis. Non-traversable slopes or fixed objects are to be removed, redesigned, relocated, delineated, or shielded by a barrier if they are within the indicated minimum clear zone width and it is cost effective to do so.
Where resources and safety benefits justify the use of clear zones, clear zone concepts may be applied to existing roads to address roadside obstacles and non-traversable terrain. Clear zones are reviewed and/or selected based on the RDG guidelines during the preparation of the location/conceptual study report.
The clear zone concept shall be applied with engineering judgment including a safety analysis (Highway Safety Manual or HSM) and be balanced with project costs. Limited right of way or high construction costs may lead to the installation of a barrier or possibly no protection at all if justified through a safety analysis. The designer should consider site specific characteristics of the roadway at issue including, if appropriate, the location of the object or area in relation to the roadway, road width, lane width, traffic volume, design speed, roadway geometry, and the crash history (including severity) involving the roadside object or area when analyzing whether or not to install a barrier. Documentation of decisions regarding clear zone strategies shall be included in the project conceptual study report, project core team minutes, or design exception forms, and shall be filed in eProjects.
Maintenance staff may remove trees, boulders, or other obstacles on existing roads where right-of-way is available. This cleared area shall not be assumed a designed clear zone as used in the EPG. If a safety concern is identified during a roadway safety audit or a project core team meeting, then the safety concern should be considered and discussed with the core team in the context of project budget and scope.
231.2.2 Roadside Obstacles
Roadside obstacles may be either man-made or natural. Barrier recommendations for roadside obstacles are a function of the nature of the obstacle itself and the likelihood that it will be hit. However, a barrier should be installed only if it is clear that the result of a vehicle striking the barrier will be less severe than the crash resulting from hitting the unshielded obstacle.
Roadside obstacles that are normally shielded are listed in Table 5-2 of the RDG. While roadside obstacles immediately adjacent to the traveled way are normally removed, relocated, modified, delineated or shielded, the optimal solution becomes less evident as the distance between the obstruction and the traveled way increases. Table 3-1 of the RDG, Suggested Clear-Zone Distances, is intended as a guide to aid the designer in determining whether the obstruction constitutes an obstacle to an errant motorist.
231.2.3 For Bridges and Culverts
Where the existing roadway is to be incorporated into a newly constructed facility as part of the main roadway, the use of clear zones will be considered on individual projects. For “spot improvements” such as bridge replacements, the designer may determine to use less than the prescribed clear zone for new construction. This decision should be based on engineering judgment, giving consideration to crash history, roadway characteristics, access issues, project scope, and budget.
The use of clear zone typical sections is not applicable to small culvert replacement projects where the intent is to continue the service of the road without upgrading it. In such cases, the typical sections in the original construction are to be used except the roadway width is not to be narrower than the template of the roadway approaching the improvement.
In shallow fills and in cuts where box or pipe culvert normally require a headwall to be located in the clear zone, the location of the headwall will be determined after an appropriate safety analysis is conducted. The culvert extension, shielding, audible warnings (rumble strips), delineation and the like (or combination of these) should be considered within the scope and budget of the project.
Clear zones should be considered on ramps. Shielding may be applicable with engineering judgment at these locations. Special consideration should be given at ramp termini where stop conditions exist. It may be acceptable to leave a non-traversable slope unshielded when operating speeds that approach zero are to be expected, especially when barriers in those areas become obstacles and receive continual damage from low speed, turning, commercial vehicle traffic.
231.2.5 Unusual Conditions
In areas where topography and right of way limits preclude the construction of the full clear zone, additional grading or the acceptance of steeper slopes may be used. In these cases, the clear zone may be omitted and guardrail used at the shoulder line to shield a non-recoverable slope. Refer to EPG 606.1.3.1, Length of Need as well as RDG 5.6.4 (Length of Need). For long fill sections through a levee or reservoir project and/or other unusual circumstances, shielding may be preferred in lieu of construction of traversable slopes. This practice may also be necessary in areas where the right of way outside the traveled way is controlled by another entity and the construction of traversable roadway slopes is a competing interest to the function of the levee/reservoir/etc. Auxiliary lanes adjacent to the traveled way are to be placed within the main roadway's clear zone with no widening of clear zone. The clear zone is located adjacent to and measured from the normal edge of the traveled way. See RDG 3.3.6 for additional information on clear zones for auxiliary lanes and freeway ramps.
231.2.6 Clear Zone Perception Issues
In most cases, the use of the clear zone concept is preferable to the use of a shielding barrier. At some locations, such as very high fills, bodies of water, steep cliffs, etc., particularly those on the outside of a horizontal curve, the clear zone alone may not provide the public with a sense of security. In these circumstances, the design may be completely safe within the guidance of the RDG, but drivers may still request the area be shielded, most often with guardrail, possibly with guard cable.
At these locations, engineering judgment, which may include a safety analysis (HSM) comparing various alternatives (clear zone widths, guardrail in lieu of clear zone, and/or a combination of clear zone and shielding) may be employed to determine if there is an effective solution. Additional guidance can be found in the RDG, paragraphs 5.2 Barrier Recommendations and 5.2.1 Roadside Geometry and Terrain Features.