Factors to consider in specifying the exact layout of a barrier at a given location include lateral offset from the edge of the traveled way, terrain effects, flare rate, and length of need. (See also Art. 6.10.)
Roadside barriers should generally be placed as far from the traveled way as conditions permit, to allow motorists the best chance of regaining control and to provide better sight distance. It is desirable to maintain a uniform clearance between traffic and roadside features such as bridge railings, retaining walls, and roadside barriers. The distance beyond which a roadside object will not be perceived as an obstacle and cause a motorist to reduce speed or change position is known as the shy line offset. According to the AASHTO Roadside Guide, this distance varies with design speed as follows:
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Design speed Shy line offset
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Place the barrier beyond the shy line offset when possible, particularly for short, isolated installations. Uniform alignment reduces the possibility of snagging. Proper transition where a barrier connects to other features is essential. Short gaps between barriers should be avoided; make the barriers continuous instead. The barrier-to-obstacle distance must be greater than the expected dynamic deflection of the barrier. Where shielding an embankment, the distance from the barrier to the beginning of the down slope should generally be at least 2 ft (0.6 m), but this may vary with local conditions for soil support of the post.
