17.3.2 Applications
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; and
- Other roadside obstacles that cannot be eliminated, made breakaway or otherwise traversable, or relocated.
: shows a summary of roadside features that are commonly shielded with guard fence. The abbreviation “cz” means within clear zone.
Roadside Feature | Applications |
Terrain Features: Steep Embankment Slope Rough Rock Cut Boulders Water Body (e.g., Ponds, Detention/Retention Ponds, Lakes, and Rivers/Streams) Lateral Drop-off (e.g., Borrow Pits) Side Ditches Quarries and Pits | cz1, See Figure 17-1 cz cz, diameter exceeds 6-in cz, depth exceeds 2-ft, permanent cz & steeper than 1V:1H and depth exceeds 2-ft cz & unsafe cross section2 see chapter 24 section 7 |
Bridges : Parapet Wall/Wingwall/Bridge Rail End Area Alongside Bridges | approaching traffic, or within cz of opposing traffic approaching traffic, or within cz of opposing traffic |
Roadside Obstacles: Trees Culvert Headwall 3 Wood Poles, Posts Bridge Piers, Abutments at Underpasses Retaining Walls, Noise Wall | cz & diameter exceeds 4-in cz & size of opening exceeds 3-ft (without safety grates only) cz & cross section/area exceeds 50 in 2 cz cz & not parallel to travel-way |
Notes: | |
1. cz – Within clear zone for highway class and traffic volume conditions. 2. For preferred ditch cross sections, see Section 4.12.7. 3. For specific 4R requirements see Section 4.10; for 3R see Section 10.3 | |
When evaluating roadside design, consideration should be given to the possible removal of metal beam guard fence if it is no longer needed.
Providing cross sections that make metal beam guard fence unnecessary is desirable.
This includes providing a sufficient clear zone recovery area and re-grading ditches to the extent possible in lieu of metal beam guard fence.Gateway Monuments are considered discretionary items (i.e., they are not necessary for the safety, maintenance, and operation of the roadway). Gateway Monuments, their foundations and any of their associated appurtenances proposed on the roadside of a facility must not be positioned within the clear recovery zone, and it is desirable that any monuments, their foundations and associated appurtenances be placed at least 10-ft beyond the clear zone. If this cannot be achieved, if a more desirable alternative location is not available, and if the monument must be placed within the clear zone, it must be shielded with an appropriate crashworthy device consistent with TxDOT Roadway Standards and applications. See
for additional information.
The combination of embankment height and side slope rate may indicate barrier protection consideration as shown in
. 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 for slopes steeper than 1V:4H.
Figure 17-1: Guide for Use of Barrier for Embankment Heights and Slopes