4.10.11 Roadside Design
Of particular concern in the design process is mitigating the number of single-vehicle, run-off-the-road crashes which occur even on the safest facilities. The configuration and condition of the roadside greatly affect the extent of damages and injuries for these crashes.
Increased safety may be realized through application of the following principles, particularly on high-speed facilities:
- A “forgiving” roadside should be provided, free of unyielding obstacles including landscaping, drainage facilities that create obstacles, steep slopes, utility poles, etc. For adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as wide as practicable for the specific highway and traffic conditions;
- Use of higher than minimum design standards result in a driver environment which is fundamentally safer because it is more likely to compensate for driver errors. Frequently, a design that includes sight distances greater than minimum, flattened slopes, etc., costs little more over the life of a project and substantially increases safety and operations; and
- For improved safety performance, highway geometry and traffic control devices should confirm drivers' expectations. Unexpected situations (e.g., left-side ramps on freeways, sharp horizontal curvature introduced within a series of flat curves, etc.) have demonstrated adverse effects on traffic operations.
For existing highways, treatment of obstacles should be considered in the following order:
- Remove the obstacle.
- Redesign the obstacle so that it can be safely traversed.
- Relocate the obstacle to a point where it is less likely to be struck.
- Make the obstacle breakaway.
- Apply a cost-effective device to provide for redirection (longitudinal barrier) or severity reduction (impact attenuators). Barrier should only be used if the barrier is less of an obstacle than the obstacle it would protect, or if the cost of safety treating the obstacle is prohibitive.
- Delineate the obstacle (would require a design waiver).
These principles have been incorporated as appropriate into the design guidelines included herein. These principles should be examined for their applicability at an individual site based on its particular circumstances, including the aspects of social impact, environmental impact, economy, and safety.
See
for additional information.