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. Median barriers designed to redirect vehicles striking from either side require some different considerations from those for roadside barriers. However, performance requirements are the same as given in NCHRP 350 for roadside barriers.

Median barriers should be installed only if the consequences of striking the barrier are less severe than those of striking the feature in question. Figure 6.25 provides sug­gested warrants for median barriers on high-speed fully controlled-access roadways with relatively flat, traversable medians. The median width and the traffic volume dictate the need. There has been a trend to use median barriers for somewhat wider median widths than in the past as a result of studies of cross-median crash history. Site-specific data should also be considered. Also, special consideration should be given to barrier needs for medians separating roadways at different elevations.

The information presented in Arts. 6.6, 6.7, and 6.8 on selection, placement, and upgrading of roadside barriers applies generally to median barriers as well. Some additional information on transitions and placement follows in Arts. 6.9.2 and 6.10. End treatments are discussed in Art. 6.12.

I. Structural adequacy*

A. Longitudinal section

1. Standard barrier designf

2. Adequate post spacing

3. Rail element blocked out on strong-post system

4. Adequate splices in rail element

B. Terminal

1. Standard terminal designf

2. Adequate anchorage strength

C. Transition section

1. Standard transition designf

2. Adequate anchorage strength

3. Adequate stiffening in advance of rigid system

4. Adequate blockout and/or rubrail

II. Functional adequacy^

A. Longitudinal section

1. Adequate length to shield area of concern

2. Proper height of rail§

3. Proper flare rate

4. Barrier-to-object distance exceeds barrier deflection distance

5. Placement behind curb consistent with vehicle trajectory data

6. Placement on flat slopes (1:10) or on slopes up to 1:6 consistent with vehicle trajectory data

7. Beam backup plates present on steel strong-post system

B. Terminal

1. Adequate clear recovery area behind yielding terminal

2. Adequate offset of terminal end

*Structural adequacy is inherent in the barrier itself, rather than resulting from design, placement, or maintenance.

fStandard systems or elements are those which are currently an approved agency standard or have been successfully crash tested. Certain barriers that fall outside these categories may be left in place depending on the characteristics of the barrier and the results of an engineering analysis of the site.

^Functional adequacy results from barrier design or placement and is essential for barrier effectiveness.

§Generally, a 3-in (75-mm) variation from the nominal height is acceptable.

Source: From Roadside Design Guide, AASHTO, Washington, D. C.,

2002 and 2006, with permission.

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. After major variables are established, the length of the barrier will then depend on the tangent length L1, the distance from the traveled way L2, and the flare rate a:b. If a semirigid railing is connected to a rigid barrier, the tan­gent length should be at least as long as the transition section to reduce pocketing and increase likelihood of redirection. After variables have been selected, the required

length of need X in advance of the area of concern, for essentially straight sections of roadway, can be calculated from

La + (/a)/) – L2
(b/a) + (La/Lr)

The lateral offset Y from the edge of the traveled way to the beginning of the length of need is

R

The amount of rail installed should be a multiple of 12.5 or 25 ft (3.8 or 7.6 m), because metal-beam barriers are furnished in these lengths. A crashworthy end treat­ment must be added if the end treatment is located within the clear zone or in a loca­tion where it is likely to be struck. If the end treatment permits vehicle penetration, it must be extended upstream to preclude a vehicle from penetrating and striking the shielded feature.

Figure 6.24 shows the definition of variables of an approach barrier for opposing traffic. In this case, lateral dimensions are measured from the edge of the traveled way of the opposing traffic. This would be the centerline for a two-lane roadway or the edge of the driving lane next to the median for a two-way divided roadway. There are three ranges of clear zone width LC to consider for an approach barrier for opposing traffic:

• If the barrier is beyond the clear zone, no additional barrier or crashworthy end treatment is required.

• If the barrier is within the clear zone but the area of concern is beyond it, no addi­tional barrier is required but a crashworthy end treatment should be used.

• If the area of concern extends well beyond the clear zone, consider shielding only that portion which lies within the clear zone (set LA equal to LC).

The lateral placement of the approach rail should satisfy the criterion for embankment slopes. If steeper than 1:10, consider flattening the slope or decreasing the flare rate so the embankment criterion is not violated.

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 parapet or railing, for example. However, the greater the flare rate, the greater the potential impact angle and the severity of an accident if the barrier is hit. Also, the chance that a vehicle would be redirected across the roadway increases. Maximum flare rates depend on design speed, barrier type, and location relative to the shy line as shown in Table 6.4. Adjustment to a flatter rate is sometimes made to avoid extensive grading.

Flare rate for
barrier beyond shy line