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25 CHAPTER FIVE CONCLUSIONS AND RESEARCH NEEDS (813 mm) to 57 in. (1,448 mm), with 32 in. (813 mm) and 42 in. (1067 mm) being the most common. ⢠For overhead sign supports mounted on median barri- ers, lateral widening of the barrier in the vicinity of the support is the most common treatment used to reduce the severity of ZOI impacts. ⢠A wide variety of post types and techniques are used to mount small sign supports on rigid median barriers; consequently, various degrees of yielding occur during ZOI impacts with the supports. ⢠Eliminating the need for a sign support by moving the sign is the most common ZOI remediation treatment for small sign supports mounted on rigid median barriers. ⢠Treatments that are specially intended to reduce ZOI exposure are used less often for small sign supports than for overhead sign supports mounted on rigid median barriers. Responses from the survey questionnaire indicate that tens of thousands of barrier-mounted sign supports are cur- rently installed in the United States. In most cases, these signs are essential to convey critical information to driv- ers; however, it is not essential that all of them be mounted on the median barrier. Many of these signs were installed before concerns over ZOI implications were identified, in the general belief that objects mounted on top of barriers were outside the vehicle impact zone and therefore not a safety concern. This study seeks to clarify the current national sta- tus of sign supports mounted on median barriers in order to begin the process of assessing the relative risks associated with their placement. One of the issues presented in the problem statement for this study is that only limited guidelines are available to help highway designers assess the need for and then design barrier-mounted sign supports. Chapter two provides a summary of pertinent sections of national design manu- als and guidelines that address this subject. In general, these documents identify ZOI issues and discourage the haphazard use of barrier-mounted signs with little regard for impact performance (FHWA 2009; AASHTO 2011). However, specific guidance is lacking on which rigid bar- rier types adequately accommodate attached sign supports when signs mounted within the ZOI might be deemed necessary. Some of the data required to establish these guidelines are not currently available; thus, many of these This synthesis study was conducted to identify and report on the current state of practice for mounting permanent highway signs on top of or within rigid median barriers throughout the United States. Information related to design standards, guidelines, individual agency practices, and research was gathered and evaluated to assess the extent to which barrier- mounted signs are used and the level of consideration given to potential safety concerns with some practices. The concept of a zone of intrusion is used to describe an area above and behind the face of a rigid barrier system where a substantial part of a vehicle can pass through during an impact event. Fixed objects placed within a barrier ZOI have been shown to reduce the crashworthiness of the barrier in the vicinity of the object (Keller et al. 2003; Caldwell 2011). The size and shape of the ZOI varies among the wide variety of rigid median bar- riers currently in service throughout the nation. In addition, the ZOI configuration for a particular barrier design varies with impact conditions. For example, the width and height of the ZOI for a single-unit truck impact would be significantly greater than those for a pickup truck impact into the same bar- rier. Likewise, the ZOI for small passenger car impacts might be minimal, whereas it could be significant for vehicles with a higher center of gravity impacting the same barrier system. Thus, the ZOI is a varying characteristic that depends on bar- rier design and impact conditions. CONCLUSIONS The following conclusions can be drawn from this synthesis study, and the subsequent discussion provides further details and commentary. ⢠Currently, tens of thousands of sign supports are mounted on rigid median barriers installed throughout the United States. ⢠Existing national design guidelines identify some issues related to vehicle impact ZOI; however, limited specific guidance is available to address the wide range of barrier types and sign supports currently in use. ⢠A limited number of treatments, with varying degrees of effectiveness, have been used to reduce the severity of ZOI impacts with barrier-mounted sign supports. ⢠NJ safety-shape, F-shape, single-slope, and vertical wall are the most common rigid barrier shapes used in the United States. Barrier heights range from 32 in.
26 decisions must be made on a case-by-case basis by indi- vidual designers. In addition to explaining the ZOI and its potential effects on barrier performance, the latest edition of the AASHTO Roadside Design Guide describes a number of treatments that can be used to reduce ZOI exposure (AASHTO 2011). These treatmentsâdiscussed in chapters two and three of this reportâhave been employed by transportation agen- cies to varying degrees. Because it is often impractical to locate a sign completely outside a barrierâs ZOI for all impact conditions, these treatments can be effective in reducing intrusion at sign locations and thus reducing the severity of those impacts. Responses to the survey questionnaire show that four rigid barrier shapes are used predominately throughout the United States: NJ safety-shape, F-shape, single slope, and vertical wall. The nominal height of these barriers ranges from 32 in. (813 mm) to 57 in. (1,448 mm), with 32 in. (813 mm) and 42 in. (1,067 mm) being the most common. For rigid barri- ers, height is the primary factor that affects the ZOI for TL-3 and higher impact conditions. In general, higher barriers reduce vehicle roll during an impact and thus reduce the size of the ZOI by limiting overhang of the upper portions of the vehicle over the barrier. The shape of the barrierâs face has some effect on the ZOI; this effect likely varies with overall height. However, definitive relationships between shape and ZOI have not yet been established. Responses to the survey questionnaire related to overhead sign supports showed that 22 of the 51 agencies reported having at least 100 overhead sign supports mounted within or on top of rigid median barriers. A majority use round pipe supports supported by foundations that are either built as an integral part of the barrier or constructed in between barrier half-sections. Lateral widening of the barrier cross-section is the most common technique used to reduce ZOI expo- sure for overhead sign supports. Often, widening the barrier in the vicinity of the support is needed to accommodate its foundation. Current guidelines recommend maximum lat- eral flare rates of the barrier for various applications to avoid abrupt bulges in the barrier (AASHTO 2011). A majority of the responding agencies try to eliminate the need for over- head sign supports in the median. Information related to small sign supports mounted on median barriers is somewhat limited compared with infor- mation on overhead supports. Nineteen of the 51 responding agencies could not estimate the number of signs within their jurisdiction, while 15 agencies reported having at least 100, and six reported having more than 1,000. There is a wide variety of mounting configurations, and they vary signifi- cantly in level of structural integrity. Thin-walled tubular posts are used for very small signs, and many are expected to yield if struck during a ZOI impact. For larger signs, substantial structural members are used for supports; the heaviest are expected to act as true fixed objects under a wide range of ZOI impact conditions. Eliminating the need for barrier-mounted supports is the most common method of reducing ZOI exposure for small signs. Ten agencies reported having existing breakaway supports on barrier- mounted signs, but no details of these installations were provided, indicating that they may no longer be in use for new signs. Several agencies said they use higher barriers in urban areas, where barrier-mounted signs are more likely to be needed, and that barrier height sufficiently addresses their ZOI concerns. RESEARCH NEEDS ZOI crash testing and research for sign supports mounted on median barriers have increased in recent years. Earlier research established the ZOI concept and recognized it as a potential hazard for a variety of objects located on or near rigid barriers (Phillips and Bryden1984; Keller et al. 2003; Wiebelhaus et al. 2008). Subsequent research has used both computer simulation and full-scale crash test- ing to evaluate specific combinations of sign supports and barrier designs (Reid and Sicking 2010; Caldwell 2011; Williams and Menges 2011; Abu-Odeh et al. 2013). There is a need for additional design, testing, evaluation, and acceptance of sign support/barrier combinations that cover a wide range of barrier and sign configurations. A set of design options that covers a wide range of sign sizes and barrier types could ultimately be included in national guidelines, which could lead to standardization among transportation agencies. Additionally, transportation agencies would benefit from research directed toward defining ZOI characteristics for common barrier designs with different shapes, widths, and heights. This information could be used to develop guide- lines that suggest which barrier designs better accommodate sign supports mounted on them, based on safety perfor- mance. The results could also be used to establish minimum lateral offset distances from the face of the barrier to the edge of the sign support for common barriers. In addition, it would be beneficial to define acceptable and unacceptable levels of intrusion and identify components of the impacting vehicle that cause more occupant risk when they strike an object in the ZOI. There may also be a need for performance evaluations to determine the extent of any safety problem associated with existing installations and to evaluate the effectiveness of current treatments used to mitigate ZOI exposure. These tests could also be used to evaluate the relative risks of sign versus no-sign design options. Information gathered from these studies would provide valuable input for cost/benefit analyses to identify cost-effective design options.
27 Additionally, there may be a need to evaluate possible effects on driver expectations and behavior resulting from implementation of treatments intended to reduce ZOI exposure. Modifications to signs, supports, and locations might impair visual recognition and thus reduce their effectiveness in conveying vital information to drivers. This could be an important factor in evaluating the full range of safety issues related to median-barrier-mounted signs. This synthesis study identified a wide variety of practices currently in use throughout the United States. Some designs incorporate treatments intended to improve the safety per- formance of sign supports mounted on median barriers, while others follow older standards that were in use before the concept of the ZOI was established. The study results indicate the need for additional research to help highway designers develop unique and standard details for mounting permanent signs on rigid median barriers.
