Care must be taken not to install a noise barrier in such a way that it will be a safety hazard. The general considerations presented in Chap. 6, Safety Systems, apply here. Noise barrier design should incorporate all of the safety design techniques used in the basic roadway design. Examples of features that should be considered include transverse location to provide required clear zone, slopes of berms, sight distances, wall ends, plantings, and transitions.
Ideally, noise barriers should be located beyond the clear zone. If not, a traffic barrier may be warranted. It is usually best to design the traffic barrier as part of the noise barrier. If a wall is located at or near the edge of the shoulder, the portion of the wall above the traffic barrier should be capable of withstanding the force of an occasional vehicle that may ride up above the top of the barrier. Concrete or masonry construction would often be used in this case. However, laminated wood construction may also be used.
At locations such as ramps, intersections, and merge areas, care must be taken to avoid blocking the line of sight between vehicles. The AASHTO Guide on Evaluation and Abatement of Traffic Noise gives the following suggestions for placement of noise barriers:
For on and off ramps, the minimum set back of a noise barrier is based upon the stopping sight distance, which is a function of the design speed and radius of curvature of the ramp. For ramp intersections, proper barrier location is set by the sight distance corresponding to the time required for a stopped vehicle to execute a left-turn maneuver (approximately 7.5 s). For intersecting roadways, barrier placement is determined from stopping sight distance, which depends on driver reaction time and deceleration rate.
The AASHTO Guide Specifications for Structural Design of Sound Barriers indicate that, when locating a sound barrier near a gore area, the wall should begin or end at least 200 ft (60 m) from the theoretical curb nose location.
Protrusions that could constitute a hazard must be avoided near traffic lanes, as well as facings that could become missiles in the event of a crash. Also, surfaces must not create excessive glare.
Sometimes it is necessary to store plowed snow between the roadway and the barriers over a width of 6 to 10 ft (1.8 to 3.0 m). In such cases, it should be removed as soon as practical to avoid blowing on the roadway and freezing. Also, there has been some occasional damage to wall panels from the pressure created by snowplows, and this should be avoided as well. Aside from snow storage, highway engineers should consider the potential for roadway icing problems resulting from deep shadows cast by walls.
The end of a noise barrier or earth berm can be a hazard to approaching traffic. When exposed to approaching traffic within the clear zone area, it should be treated with protection similar to that for other fixed objects. Barrier rails or crash cushions may be appropriate. End slopes for earth berms should be 6:1 or flatter, with 10:1 or 15:1 desirable.
