17.6.2 Design Equations

To determine the needed length of guard fence or barrier required for a given obstacle, design equations have been formulated for low volume (ADT 750 or less) and higher volume (ADT more than 750) conditions. A clear zone width of 16-ft and length of roadside travel of 200-ft are incorporated in the low volume design equation (for use on roadways when the present ADT volume is 750 or less). Also, if the clear zone required is less than 16-ft and the present ADT is 750 or less, use

Equation 17-1

for calculating the guard fence length of need.

Table 17-2: Equations for Upstream and Downstream Length of Need
ADT ≤ 750	$L_{u} = 200 - \frac{200}{D_{u}} x G_{u}$ Equation 17-2
	$L_{d} = 200 - \frac{200}{D_{d}} x G_{d}$ Equation 17-3
ADT > 750	$L_{u} = 250 - \frac{250}{D_{u}} x G_{u}$ Equation 17-4
	$L_{u} = 250 - \frac{250}{D_{d}} x G_{d}$ Equation 17-5

Where:

L_u = Length of guard fence needed (upstream of area of concern), ft

L_d = Length of guard fence needed (downstream of area of concern), ft

D_u = Distance from edge of travel lane to far side of area of concern or to outside edge of clear zone, whichever is least, ft (for upstream direction of traffic)

D_d = Distance from edge of travel lane to far side of area of concern or to outside edge of clear zone, whichever is least, ft (for opposing direction of traffic)

G_u = Guard fence offset from edge of travel lane adjacent to proposed guard fence, ft

G_d = Guard fence offset from edge of opposing direction of travel lane (centerline), ft

For low volume conditions, if the clear zone width of 16-ft is met or exceeded, L=0.

For higher volumes, a clear zone width of 30-ft and length of roadside travel of 250-ft are incorporated into the design equation (for use on roadways when the present ADT volume is more than 750)

For high volume conditions, if the clear zone width (30-ft) is met or exceeded, L=0.

17.6 Determining Length of Need of Barrier