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Section 2: Barrier Need

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Overview

Traffic barriers are considered only when the obstacle is less forgiving than striking the barrier itself.

Should a roadside obstacle exist, treatment should be considered in the following priority:

  1. Eliminate the obstacle.
  2. Redesign the obstacle so it can be safely traversed.
  3. Relocate the obstacle outside the obstruction free zone to reduce the likelihood that it will be struck.
  4. Treat the obstacle to reduce accident severity, i.e., use flush or yielding designs.
  5. Shield the obstacle with a barrier (median barrier, roadside barrier, or crash cushion).
  6. Delineate the obstacle if the above alternatives are not appropriate.

The three basic types of obstacles that are commonly shielded using roadside barriers are as follows:

  • slopes, lateral drop-offs, or terrain features
  • bridge ends and the areas alongside bridges
  • other roadside obstacles that cannot be eliminated, made breakaway or otherwise traversable, or relocated.

Table A-1 shows a summary of roadside features that are commonly shielded with guard fence.

Anchor: #CHDDEJIDTable A-1: General Applications of Conditions for Roadside Barriers

Roadside Feature

Applications

Terrain Features:

Steep Embankment Slope

cza, See Figure A-1

Rough Rock Cut

cz

Boulders

cz, dia. Exceeds 6 in [150 mm]

Water Body

cz, width exceeds 2 ft [600 mm], permanent

Lateral Drop-off

cz & steeper than 1V:1H and depth exceeds 2 ft [600 mm]

Side Ditches

cz & unsafe cross sectionb

-

Bridges:

Parapet Wall/Wingwall/Bridge Rail End

approaching traffic

Area Alongside Bridges

approaching traffic

-

Roadside Obstacles:

Trees

cz & dia. Exceeds 6 in [150 mm]

Culvert Headwall

cz & size of opening exceeds 3 ft [900 mm] (w.o. safety grates only)

Wood Poles, Posts

cz & cross section/area exceeds 50 in2 [32000 mm2]

Bridge Piers, Abutments at Underpasses

cz

Retaining Walls

cz & not parallel to travelway

a cz - Within clear zone for highway class and traffic volume conditions.

b For preferred ditch cross sections, see Side Ditches in Chapter 2



The combination of embankment height and side slope rate may indicate barrier protection consideration as shown in Figure A-1. For low fill heights a more abrupt slope rate is tolerable than at high fill heights. Because steeper than 1V:4H side slopes provide little opportunity for drivers to redirect vehicles at high speeds, in the absence of guard fence, a 10 ft area free of obstructions should be provided by the designer beyond the toe of slope.

(US). Guide for Use of Guard fence for
Embankment Heights and Slopes (US Customary) (click in image to see full-size image) Anchor: #i1001652grtop

Figure A-1. (US). Guide for Use of Guard fence for Embankment Heights and Slopes (US Customary)

NOTE: Online users can view the metric version of this figure in PDF format.

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