Section 2: Barrier Need
Anchor: #i1005219Overview
Traffic barriers are needed only when the obstacle without the barrier is greater than the obstacle of the barrier itself.
Should a roadside obstacle exist, treatment should be considered in the following priority:
- Eliminate the obstacle.
- Redesign the obstacle so it can be safely traversed.
- Relocate the obstacle outside the obstruction free zone to reduce the likelihood that it will be struck.
- Treat the obstacle to reduce accident severity, i.e., use flush or yielding designs.
- Shield the obstacle with a barrier (median barrier, roadside barrier, or crash cushion).
- 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 guardrail.
Anchor: #CHDDEJIDTable A-1: General Applications of Conditions for Roadside Barriers Roadside Feature
Applications
Terrain Features:
Steep Embankment Slope
hca, See Figure A-1
Rough Rock Cut
hc
Boulders
hc, dia. Exceeds 6 in [150 mm]
Water Body
hc, width exceeds 2 ft [600 mm], permanent
Lateral Drop-off
hc & steeper than 1V:1H and depth exceeds 2 ft [600 mm]
Side Ditches
hc & unsafe cross sectionb
-
Bridges:
Parapet Wall/Wingwall/Bridge Rail End
hc & approaching traffic
Area Alongside Bridges
hc & approaching traffic
-
Roadside Obstacles:
Trees
hc & dia. Exceeds 6 in [150 mm]
Culvert Headwall
hc & size of opening exceeds 3 ft [900 mm] (w.o. safety grates only)
Wood Poles, Posts
hc & cross section/area exceeds 50 in2 [32000 mm2]
Bridge Piers, Abutments at Underpasses
hc
Retaining Walls
hc & not parallel to travelway
a hc - Within horizontal clearance for highway class and traffic volume conditions.
b For preferred ditch cross sections, see Side Ditches in Chapter 2
Where the prescribed length of the guardrail cannot be installed at a bridge end due to an intervening access point such as an intersecting roadway or driveway, the length of guardrail 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 wrapping the guardrail around the radius of the access location, terminating the guardrail prior to the access location with an appropriate end treatment and continuing the guardrail beyond the access location if necessary or using an alternate bridge end treatment. The selected treatment should consider potential sight line obstructions, cost and maintenance associated with the selected treatment and any accident history at the site. Reduced guardrail length to accommodate access points will not require a design exception or a design waiver.
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 guardrail, an area free of obstructions should be provided by the designer beyond the toe of slope.
Figure A-1. (US). Guide for Use of Guardrail for Embankment Heights and Slopes (US Customary)
Figure A-2. (M). Guide for Use of Guardrail for Embankment Heights and Slopes (Metric)