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Noise Barrier Design Handbook

9. Safety Considerations

This section describes the various safety considerations related to noise barrier design. Safety is a factor which must be given appropriate consideration in the design of any noise barrier system. While organization and/or industry standards are adhered to in the design of noise barriers, these standards are usually geared to providing acceptable designs related to factors such as dead loads, overturning loads, durability, strength of materials, etc.

9.1 Qualitative Evaluation of Safety

Barriers should be located (where possible) so as to be somewhat protected from vehicular impact, especially since they are not specifically designed to withstand severe collisions. In addition, vehicles should be provided the appropriate protection against impacting a barrier. Since no known standards or formulas exist to specifically determine "how safe is safe" in terms of noise barrier design associated with impacts, decisions related to determining the "appropriate" degree of protection to provide in the construction of barriers in any given situation may be based on the "best engineering judgment" of the designer. Unfortunately, but understandably, such decisions are sometimes influenced by (and in some cases totally based on) individual opinions and concerns, but with little or no quantitative or qualitative assessment of risks, probability of impact occurrence, or other evaluations.

Although a topic requiring research attention, the development of a quantitative formula or standard related to the above discussed issues is beyond the scope of this Handbook. However, a qualitative evaluation with the consideration of the following factors is considered imperative in the design of noise barriers where safety concerns exist.

9.1.1 Evaluate the Need for Special Considerations Related to Safety.

The first step in the qualitative evaluation process should involve determining whether or not any special modifications to the normal noise barrier design are warranted. Things to consider in this determination include, but are not limited to:

9.1.1.1 Probability of Occurrence of the Barrier Being Impacted.

In this context, "impact" should be considered a force sufficient to result in the barrier either being knocked down or shattered to the point where the resultant debris becomes a safety concern. One should investigate the history of barriers being impacted in similar orientations elsewhere (nationally and/or statewide) and evaluate, based on the known accident history of the particular highway (if barrier is to be placed on an existing highway) or similar area highways (similar volumes, composition, configuration, etc.), the probability of impact.

9.1.1.2 Consequences of the Barrier Being Impacted.

Assuming that the evaluation in Section 9.1.1.1 concludes that a high enough probability of occurrence exists (where "high enough" is defined by the responsible organization) to warrant further special considerations, an evaluation of the consequences of barrier impacts should be undertaken. In areas where a ground-mounted noise barrier is proposed, the area immediately behind the noise barrier should be evaluated. If this area happens to be an actively used portion of a backyard, a school playground, or frequented area, the consequences of impact will be greater than in portions of such properties with little or no regular activity (see Figure 211). In areas where a noise barrier is proposed to be erected on a bridge or viaduct section, the frequency and type of use beneath and adjacent to the bridge should be identified. Fewer consequences are likely in areas where the barrier passes over or adjacent to low volume roadways or relatively inactive lands as compared to high volume roadways and active land uses. Consideration should also be given in this evaluation to the added protection provided by the barrier in terms of retaining objects (vehicle parts, vehicle cargo, stones, road salts, etc.) which, in the absence of the barrier, would pose a potential hazard to land uses adjacent to or beneath the highway. While this discussion has focused on the consequences to adjacent land users, the consequences of barrier impact need to also be evaluated in terms of the drivers and passengers of vehicles operating on the highway.

Figure 211. Consequences of a barrier being impacted
^{photo #1148}

9.1.2 Modifications to the Noise Barrier Design.

Assuming that the evaluation in Section 9.1.1.1 indicates that sufficient consequences exist to warrant further special considerations, modifications to the standard noise barrier design should be evaluated. Such modifications may relate to the barrier's location, attachment/reinforcement details, type, and protective devices as discussed below. As each modification is evaluated, a re-evaluation of the applicable factors in Section 9.1.1 should be performed.

• Barrier location - The location of the barrier may be able to be modified to place it in a position where it is less vulnerable to impact by vehicles. If conditions (topography, acoustic requirements, drainage, etc.) permit, a ground-mounted barrier at the property line of an active land-use area may be moved closer to the highway. If structural conditions permit, a noise barrier designed to sit on top of the bridge parapet may be repositioned to be mounted on the outside face of the parapet, giving added protection from potential impacts of automobiles and trucks.
• Barrier attachment/reinforcement details - Barrier component attachment and/or reinforcement details may be modified. As shown in Figure 212, cables can be placed through pre-drilled/pre-formed holes in posts and either run through horizontal conduits in concrete and composite panels or attached to panels in metal and wood systems. These cables thus tie a barrier system together and provide a means of retaining pieces of a barrier damaged by a vehicular impact. Additional reinforcement rods or mesh can be added to concrete, masonry block and brick noise barriers to increase the panels' strength and reduce the size of shattered pieces. Similarly, additional framing can be added to wood, metal, and composite barrier systems. Such additions add weight to the barrier system, a factor which could become a problem on structure-mounted barriers. In addition, they could also compromise barrier aesthetics.

  

  Figure 212. Barrier attachment/reinforcement details
  ^{photo #1267}

• Barrier type - The type of barrier may be modified in terms of its material type and/or configuration. Certain barriers can be designed without posts or with concealed posts. These "post-less" systems reduce the potential for errant vehicles to "snag" a protruding post. In many cases, lighter-weight materials have been selected for barriers located in areas deemed to be of higher risk to adjacent land users. This is particularly true on structures, where such lighter-weight systems have an obvious advantage in terms of structure load designs. The somewhat common conception that lighter-weight barriers will be less likely to cause damage if knocked down or if knocked off a bridge requires a more rigorous evaluation. Such systems can still require quite heavy metal, wood, or concrete posts as well as have components which, if dislodged, can tear and "sail" larger distances than heavier panel components. Lighter-weight barrier systems tend also to be more susceptible to damage from more frequent, albeit less major, impacts from vehicular scrapes, vehicle parts, vehicle cargo, stones, road salts, etc. The potential exists for such lighter weight panels to be knocked down by a collision which could have been otherwise withstood by a heavier, more substantial barrier. Obviously, the maintenance and durability implications of any barrier modification need to be thoroughly evaluated in the decision-making process.
• Barrier protective devices - In addition to the considerations listed above, the potential for a noise barrier being impacted can be reduced by the placement of a protective barrier (steel guard rail, concrete Jersey barrier, etc.) between the noise barrier and the highway traffic or by erecting a higher than normal safety shape barrier in front of a noise barrier in close proximity to traffic. Since protective barriers are themselves considered an obstruction, any such protective barrier must be designed in compliance with appropriate standards. In addition, the consequences of impact of a vehicle hitting the protective barrier must be weighed against impact consequences which would exist in the absence of such a protective device. Section 9.3 provides a more detailed discussion on protecting a barrier from traffic.

9.1.3 Overall Results of Qualitative Evaluation .

By objectively considering the above factors, a rational decision regarding the appropriate consideration of safety issues and concerns can be made consistent with the goals and criteria related to acoustical, structural, and aesthetic aspects of noise barrier design. Through such an evaluation, the appropriate justification can be documented and the appropriate safety-related noise barrier modifications can be implemented on a case-by-case basis.

9.2 Sight Distance

Sight distance (as impacted by noise barriers) is a factor requiring consideration along horizontal curve sections of highways and at locations where a barrier terminates near a highway's or ramp's intersection with another roadway. Noise barriers (either wall or berm) placed in either the median or along the outside of a highway can impact sight distance. Solutions to providing adequate sight distance include additional setback of the barrier, curving the barrier back approaching intersection (see Figure 213), or terminating the barrier at a point short of its required acoustical end point.

Figure 213. Sight distance
^{photo #612}

9.3 Traffic Protection

Noise barriers are protected from vehicular impacts when constructed within the recovery zone (clear zone) normally provided for vehicles. Devices used to provide such protection include metal or wood guard rails (or guide rails) or concrete safety shaped (Jersey barriers) protective barriers. Metal and wood guardrails are placed at a distance in front of the noise wall equal to or greater than the maximum deflection of the guard rail (see Figure 214). Concrete Jersey barriers are sometimes placed immediately in front of a noise barrier, but are often placed some distance in front of the noise wall with the area between the Jersey barrier and the noise barrier sometimes filled with stone or earth. This latter treatment provides a greater degree of protection and also allows for plantings in the area between the Jersey barrier and the noise barrier (see Figure 215). Certain cast-in-place concrete and post and panel noise wall systems incorporate the Jersey barrier shape in the bottom portion of the noise wall as either an integral portion of the noise wall or as the bottom panel of the noise wall (see Figures 216 and 217). Where the noise wall is potentially subject to vehicular impact on both sides (such as where a parallel frontage road or a local perpendicular street exists) such protection may be required on both sides of the noise barrier.

Figure 214. Traffic protection
^{photo #1251}

Figure 215. Traffic protection
^{photo #64}

Figure 216. Traffic protection
^{photo #4}

Figure 217. Traffic protection
^{photo #8036}

9.4 Emergency Access

Noise barriers often interrupt the path between the highway and adjacent local roadways. During emergencies (accidents, spills, fires, etc.) access from these local roadways is often necessary and/or desirable, particularly on stretches of limited access highways with significant distances between interchanges. Barrier access points for emergency or maintenance situations are typically identified with signs on both sides of the barrier to enable the proper coordination of emergency/maintenance personnel. Such signs may be located either on the barrier itself and/or along the highway and the adjacent roadway.

9.4.1 Barrier Overlap Sections.

Access ways can be provided by overlapping barriers (see Figures 218 and 219. The distance between the walls of the overlapping section (measured perpendicular to the barriers) is typically dictated by the size of equipment which may need to pass through the access opening; whereas, the length of the overlap is almost always a function of acoustics such that the overlap length is designed to maintain the amount of noise leak at an absolute minimum. Section 3.5.5.1 of this manual discusses the acoustical concerns related to barrier overlap sections. Another consideration related to the design of barrier overlap sections is the potential for increased crime in the immediate areas surrounding the overlapping sections, particularly where a pedestrian overpass is also located nearby. To address this concern, safety measures, including additional lighting or a modified overlap design to provide more open visibility, may need to be implemented..

Figure 218. Barrier overlap sections
^{photo #109}

Figure 219. Barrier overlap sections
^{photo #382}

9.4.2 Access Doors.

Access needs can also be met by providing doors in noise barriers at appropriate intervals. These doors need to be designed so as to be acoustically "sealed" when closed. When not in use, doors are almost always locked to avoid unauthorized access. Several techniques are used to enable authorized users to gain access through doors. They vary from combination locks to common (master) key systems. Materials used to make doors are usually either metal or wood. As such, the door's material may be different from the material of the overall barrier. This fact suggests that the noise wall designer should pay particular attention, during the design stage, to aesthetics of the access doors in relationship the remainder of the barrier (see Figures 220 to 225).

Figure 220. Access door
^{photo #819}

Figure 221. Access door
^{photo #1631}

Figure 222. Access door
^{photo #6519}

Figure 223. Access door
^{photo #1539}

Figure 224. Access door
^{photo #2344}

Figure 225. Access door
^{photo #2414}

On rare occasion, it may be necessary to design a noise wall section or sections to be temporarily removed. An example of such a situation could be related to a noise wall constructed next to a utility sub-station where the only access for large equipment is from the highway along which the noise wall runs. Noise walls can be constructed with their panel lifting inserts left intact and with sufficient space between posts to accommodate the necessary passage of equipment, repair parts, etc (see Figures 226 and 227).

Figure 226. Emergency access opening
^{photo #5017}

Figure 227. Emergency access opening
^{photo #2290}

9.5 Fire Safety

Noise barriers may also interrupt the path between the highway and a source of water required to be accessed in the event of a fire or spill on the highway. Such water sources may be a pond, lake, stream, or fire hydrant. Since fire hoses cannot be practically draped over a noise barrier, special design considerations are required. Where neither barrier overlaps nor access doors (see Figure 228) are available for running fire hoses, emergency access openings or valves can be incorporated directly into the design of the noise wall panels.

Figure 228. Access for fire
^{photo #1416}

The following is a brief description of some of the more successful designs used to date.

• Hose Couplers Incorporated into Noise Barrier Panels - This technique involves incorporating one or more couplers directly into the noise barrier panel (see Figures 229 and 230). This fixed coupler allows the connection of fire hoses on both sides of the noise barrier, and effectively eliminates any kinks in the hose.

  

  Figure 229. Access for fire
  ^{photo #54}

  

  Figure 230. Access for fire
  ^{photo #260}

  • A critical consideration with this type of a treatment is to have the correct size connection (diameter, thread size, etc.) for all fire companies which may need to access the connection. Also, strength of the panel, the adjacent posts, and, in particular, the post to panel connections must be analyzed to assure that they are capable of withstanding the thrust loads generated by the force of the moving water in the hose lines.
• Panel Mounted Valves - Another technique is to include a suitable size valve into the panels when they are being fabricated (see Figure 231). This method should also be structurally evaluated to ensure the capability of withstanding thrust loads. Another concern that should be considered with this type of system is that the closure caps (required to keep debris and other objects out of the connections and to protect the coupling threads) and their retaining chains, being brass, can become the target of theft and vandalism. Where elevation differences occur, the use of dry stand-pipes may be required (see Figure 232)

  

  Figure 231. Access for fire
  ^{photo #8089}

  

  Figure 232. Access for fire
  ^{photo #1714}

• Small Covered Openings - Another treatment involves casting an opening of sufficient size (typically about one foot square) to allow the pass through of several fire hoses. Such a treatment is compatible with any size hose and also allows for easy communications between emergency personnel on either side of the barrier. Passage of small tools (wrenches, axes, etc.) from one side of the barrier to the other is also enabled. This type of treatment requires some form of device (door, flap, etc.) to seal the opening when it is not in use in order to avoid acoustic degradation and to restrict the passage of small animals, etc. While early designs of such closure devices experienced vandalism and theft problems, recent designs have proven to be adequate. They include flaps, knock-out sections cast directly into the panels, and closures requiring special tools to open (see Figures 233 and 234).

  

  Figure 233. Access for fire
  ^{photo #1568}

  

  Figure 234. Access for fire
  ^{photo #1112}

Whichever system is used, identification signs are required on both sides of the barrier to enable the proper coordination of emergency personnel. Such signs may be located either on the barrier itself and/or along the highway and the adjacent roadway. The choice of an opening in the wall versus a valve through the wall is usually a determination made with significant input from the local fire departments.

9.6 Glare

Glare is generally a problem on noise barriers with smooth surfaces, such as metal and transparent barriers (see Figure 235). It is more prevalent on lighter colored surfaces and can be a problem in daytime (low sun angle) and nighttime (due to headlights) periods and is particularly bad during nighttime periods when the barrier may be wet. Use of rougher types of surface treatments and deep relief patterns can reduce or eliminate glare impacts.

Figure 235. Glare
^{photo #392}

Transparent noise barrier panels, or any other high gloss finish on panels, such as plastics and metals, reflect light. In unfavorable situations, by day or night, the reflection of light from these glossy surfaces can be troublesome and even dangerous for drivers, either because of glare or simply by the reflection of another vehicle's image on the glossy surface. These problems arise, in particular, in the case of curved roadways or where the traffic passes close to the wall, resulting in a low-angle incidence of light.

Consequently, in constructing high-gloss surfaced noise barriers, every precaution must be taken to ensure that such reflections do not reduce the safety of the roadway.

• Sources of Reflections:

  By Day - During the day, the reflections are produced typically by the sun, and its effects are most harmful when it is low on the horizon, namely in the morning and in the evening. The sun's rays, bounced back onto traffic, may momentarily blind a driver. This phenomenon, though of short duration, is bound to occur and its intensity is typically such that it cannot be neglected.

  By Night - The major source of reflections during night-time is normally from vehicle headlights, whose rays generally strike the reflective panels at a low angle of incidence. In an urban environment, the barriers are often situated within infrastructures where the roadway is illuminated and traffic flow is controlled by lighted signs and traffic signals. Reflections could also be coming from adjacent roadways as well.

  These reflections are directly affected by the angle of incidence. If the angle is less than 40 degrees, approximately 8 percent of the intensity of the incident beam is reflected. This is generally the case for vehicles close to the noise barrier. If the angle of incidence is greater than 40 degrees, the reflection has virtually the same intensity as the incident beam. This occurs more on curved roadways.

  To overcome some of the light reflection problems, the following solutions should be considered:

  Tilted Walls - By tilting the entire wall between 6 and 20 degrees, the reflected rays can be deflected away from the oncoming traffic, thus removing the potential hazard caused by the glare. The most effective angle must be determined on a site-by-site basis.

  Tilted Stacked Panels - An alternative is to construct the wall system using a stacked panel arrangement and only tilting the individual panels. This will provide the same results as tilting the entire wall. It may also be possible to only tilt specific portions of the wall. Tilted stacked panels are also referred to as "clapboard."

  Protrusions - This solution requires that the panels be mounted on the posts in such a way that either the posts or a similar deep protrusion will, at some distance, hide the surface of the panels from the traffic.

  Anti-glare Screens - Another option to constructing a tilted noise barrier is to install anti-glare screens in the median to remove the problem of glare from the oncoming traffic. However, dependent on the type of material used, the screen may alter the acoustical characteristics of the roadway. Therefore, the desirable type of material to use in this situation would be an "expanded web" type made of either metal or plastic which would not interfere with the acoustics. This type of anti-glare screen material could also be installed adjacent to the transparent panels. Although this would provide shielding from reflected light, the expanded metal is always visible to some degree, regardless of the angle it is being viewed through, and, thus, may reduce some of the benefits of a transparent barrier unobtrusiveness for which the barrier was designed.

9.7 Shatter Resistance

When a noise barrier is impacted by a vehicle, the effect of barrier components shattering is a concern. The effect is of particular concern where the barrier is located on a structure overpassing another highway causing potential injury to nearby vehicles, pedestrians, or adjacent residents. For each of these situations, the shatter resistance of the noise barrier components should be evaluated in conjunction with the probability of vehicular impact. Differing opinions exist on allowing the components to shatter into small pieces with little or no potential for causing injury, as opposed to totally preventing the components from shattering by installing additional reinforcements.

9.8 Icing and Snow Removal

Design of noise barriers in climates subject to ice and snow conditions should consider the placement of barriers a sufficient distance from the travel way to assure adequate space for storage of plowed snow and to assure that the barrier can withstand the additional loads that may result from plowed snow being both thrown and piled up against the barrier. Barriers also will shade portions of highways at some time during the day. Depending upon the particular orientation of the barrier and its height, this shading can result in significant interference of the normal sunlight melting of ice or snow on the highway's shoulders and travel lanes. These factors should be considered in the design process so that, in critical areas, the possibilities of wide shoulders and minimal barrier heights can be incorporated.

Section Summary