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First, there is concern with the placement of a ground-mounted, wood rub-rail system under the guardrail that may cause an impacting vehicle to vault upward as a wheel contacts the timber member, thus increasing the propensity for a vehicle to override the guardrail or become unstable during redirection. This result would especially be of concern when the wood member vertically extends greater than 4 inches above the ground-line for standard 27" tall guardrail systems.

The use of 6"x8" by 10' long wood posts in the noted situations may also result in premature post fracture and reduced energy dissipation when the drop in back slope in minimized (i.e., embedment maximized). However, the use of a 1/2-post spacing could garner back some of the reduced capacity if premature post fracture occurs. For cases where the soil drop is maximized, the soil may yield and allow post rotation prior to reaching a wood post fracture condition. To reduce concerns for wood post fracture in these special situations, it may be preferred to utilize long steel posts which would remain intact and dissipate energy during displacement of the barrier system.