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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:

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  1. Remove obstacle.
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  3. Redesign the obstacle so it can be safely traversed.
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  5. Relocate the obstacle to a point where it is less likely to be struck.
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  7. Reduce impact severity by using an appropriate breakaway device.
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  9. Shield the obstacle with a longitudinal traffic barrier designed for redirection or use a crash cushion.
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  11. Delineate the obstacle if the above alternatives are not appropriate.
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Types of Barrier

Rigid: Common rigid barriers used by TxDOT are constant single slope barriers and F-shape barriers that are cast in place or precast. Concrete barriers placed in situ or are keyed into the roadway pavement are considered rigid barriers. Concrete barriers are placed primarily where little movement of the barrier can be tolerated and typically do not deform when impacted. For concrete barriers that are anchored, between 0 and 1 ft. of deflection is anticipated for a TL-3 (high-speed) pickup truck impact. Where considerable truck traffic exists or is anticipated, a TL-4 rated barrier should be considered. (Minimum height of a TL-4 barrier is 36 inches). Concrete barriers are typically used at narrow medians. The exposed ends of the barrier need to be protected by an attenuator.

Semi-Rigid: Semi-Rigid barriers commonly used by TxDOT include metal beam guard fence and both pinned and unanchored precast concrete barriers. Semi-Rigid barriers have an expected deflection of between 18" and 60" if impacted by a MASH TL-3 pickup truck under MASH test conditions. Metal beam guard fence is the most commonly used barrier by TxDOT. The current height tolerance for a new installation of an (MGS) system is 31 inches plus or minus 1 inch measured from the road surface to top of w beam rail. Metal beam guard fence is used primarily to shield roadside obstacles, such as slopes, utility poles or trees. Precast concrete barriers can be used for temporary or permanent installations. Precast concrete barrier is most commonly used in work zones to shield personnel from traffic.

Flexible: High tension cable barriers are the most commonly used flexible barriers. A cable barrier is sometimes referred to as a wire rope safety barrier. It consists of high tension steel cables mounted on weak posts with a post foundation and typical anchor terminal. Cable barriers are used as median barriers to reduce median crossover vehicle encroachments.

Additional Guidance for each barrier type is provided in subsequent sections.

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Applications

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

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  • Slopes, lateral drop-offs, or terrain features,
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  • Bridge ends and the areas alongside bridges, and
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  • 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)

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Working Width

Working width is the distance between the traffic face of the barrier before the impact and the maximum lateral position of any major part of the system or vehicle after the impact. (see Figure A-2) Working width is related to deflection, but working width takes into account the lateral position of the vehicle. Working width should be considered when placing any longitudinal barrier.

Working Width (click in image to see full-size image) Anchor: #QKKCQDGAgrtop

Figure A-2. Working Width

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