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Table 6.6 provides a checklist that can be used to review existing barrier installations and determine adequacy for either structural or functional (design or placement) causes. Factors to be considered in determining the scope and extent of upgrading include the nature and extent of the deficiency, past accident history, and the cost-effectiveness of the recommended improvement. Remember to always consider the cost-effectiveness of eliminating or relocating the shielded feature.

6.6 MEDIAN BARRIERS

Longitudinal median barriers are used to separate opposing traffic on divided high­ways, to separate local and through traffic, or to separate traffic in designated lanes...

The total length of a longitudinal barrier needed to shield an area of concern is referred to as the length of need. Figure 6.23 illustrates the variables that must be con­sidered, particularly the runout length LR and the lateral extent of the area of concern LA. The runout length is the theoretical distance needed for a vehicle that has left the road to come to a stop, measured as shown. Suggested values are given in Table 6.5 in terms of the traffic volume and the design speed. The lateral extent of the area of con­cern is the distance from the edge of the traveled way to the far side of the fixed object, or the outside edge of the clear zone LC of an embankment or fixed object that extends past the clear zone...

Ideally, at the moment of impact, a vehicle should have all wheels on the ground and the suspension system in a neutral state. Thus, terrain conditions between the traveled way and the barrier are very important. For example, curbs should be avoided and should be no higher than 4 in (100 mm) if used. In many cases, they can be located behind the barrier. Barriers are usually tested on level terrain. If installed on slopes steeper than 1:10, vehicles may go over standard barriers or impact them too low, and thus not perform as anticipated.

6.7.2 Flare Rate

Roadside barriers must be flared (must have variable offset from the traveled way) to locate the barrier terminal back from the roadway and thus to minimize drivers’ reaction to a perceived hazard near the road when approaching a bridge p...