17.6.3 Using Design Equations to Determine Length of Guard Fence

Before determining length of guard fence, the designer should assemble the following pertinent data:
  • Present ADT volume;
  • Clear zone (horizontal clearance);
  • Traffic operations (one-way or two-way);
  • Lateral and longitudinal dimension of the area of concern;
  • Shoulder width;
  • Offset distance of the area of concern from the edge of travel lane (including from the centerline markings for two-way traffic operations);
  • Approach grading, (i.e., will slopes be 1V:10H or flatter);
  • Placement location, lateral offset measured to barrier face (alongside shoulder vs. near object, flared, etc.); and
  • Presence of other nearby areas of concern which should be considered simultaneously
Once this design data has been assembled, the equation for length of guard fence can be used.
Where the prescribed length of the guard fence cannot be installed at a bridge end due to an intervening access point such as an intersecting roadway or driveway, the length of guard fence may be interrupted or reduced. This change in length is acceptable only in locations where the Department must meet the obligation to provide access and this access cannot be reasonably relocated. Alternative treatments in these situations include installing an appropriate radius rail, terminating the guard fence prior to the access location with an appropriate end treatment and continuing the guard fence beyond the access location if necessary, or using an alternate bridge end treatment. The selected treatment should consider potential sight line obstructions, crash history at the site, cost, and maintenance associated with the selected treatment. Reduced guard fence length to accommodate access points will not require a design exception or a design waiver.
Section 17.6.4
provides example problems and solutions using the design equations.
The guard fence lengths produced by the equations should be rounded up to an even length of guard fence.
In circumstances where site conditions permit, the rounded-up length of need should terminate at the end of guard fence.
Any additional length of need component available from an end attenuator should be considered an additional buffer.