After all practical means to free the roadside of obstacles have been exhausted, certain areas may remain which constitute an obstacle to errant vehicles. These areas, as illustrated in , will be referred to as an “area of concern.”

illustrates the variables of interest in the layout of approach barrier to shield an area of concern. The total length of need is equal to the sum of the following variables:

$L_{\text{total}}=L_u+L_p+L_d$

Equation 17-1

Where:

When discussing length of need as it pertains to metal beam guard fence, L

u

is the length of guard fence needed to shield the area of concern for adjacent traffic. Upstream refers to the guard fence upstream of traffic adjacent to proposed guard fence. L

d

is the length of guard fence needed to shield the area of concern for opposing traffic. For roadways serving one-way traffic operations, L

d

= 0. L

d

is greater than zero for two-way operations when the area of concern lies within the clear zone of opposing (northbound in ) traffic as measured from the centerline pavement markings

In certain instances, judgment should be exercised to supplement design chart solutions and provide for additional safety. For example, high severity fixed objects (e.g., bridge columns) may justify minimum guard fence treatment when located outside the clear zone if geometric conditions (i.e., steep fill slope, outside of horizontal curvature, etc.) increase the likelihood of roadside encroachments. Also, bridge class culverts require protection inside and outside the clear zone. If a bridge class culvert is outside the clear zone, consider increasing the offset of the metal beam guard fence to decrease the length of need. Maintain a 4-ft minimum distance away from the obstacle and provide a maximum slope of 1V:10H or flatter for the placement of the metal beam guard fence. If the bridge class culvert is either inside or outside the clear zone, Du equals the clear zone distance. Additionally, Du equals the clear zone distance for any bridge structure.