When designing or choosing a guardrail system, consider the following: lateral offset from the edge of traveled way; maximum deflection distance of the barrier; length of need; and end treatment. Guardrail performance is best when the terrain between the traveled way and the barrier is level.
The first decision in selecting an appropriate traffic barrier is the level of performance required. Test Level 3 (TL-3) barriers (in accordance with NCHRP Report 350 and MASH testing criteria) are the most commonly used systems. The type of barrier selected will depend on the maximum deflection of the guardrail if hit, the location of the roadside obstacle, and site conditions (such as steep slopes). The minimum lateral distance between a guardrail and an obstruction varies between 2 and 7 feet, depending on the type of guardrail used.