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CONTENTS 1 SUMMARY 3 CHAPTER ONE INTRODUCTION Background, 3 Scope, 6 Study Approach, 6 8 CHAPTER TWO SIGN AND BARRIER DESIGN CRITERIA AND GUIDELINES Sign Usage and Placement, 8 Types of Rigid Median Barriers and the Zone of Intrusion, 9 Techniques for Reducing Zone of Intrusion Exposure, 11 12 CHAPTER THREE EXISTING SIGN AND BARRIER COMBINATIONS Rigid Barrier Types and Usage, 12 Overhead Sign Supports, 12 Small Barrier-Mounted Sign Supports, 15 22 CHAPTER FOUR TESTING AND RESEARCH Early Research, 22 More Recent Research, 22 25 CHAPTER FIVE CONCLUSIONS AND RESEARCH NEEDS Conclusions, 25 Research Needs, 26 28 REFERENCES 29 APPENDIX A NCHRP SYNTHESIS TOPIC 44-14 SURVEY QUESTIONNAIRE Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.
SUMMARY PERMANENT SIGNS MOUNTED ON MEDIAN BARRIERS The synthesis study was conducted to identify and report on the current state of practice for mounting permanent highway signs on top of 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 some practices give to potential safety con- cerns. The concept of a zone of intrusion (ZOI) 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 dur- ing an impact event. If an object such as a sign support is attached to the barrier within the ZOI, it is likely to reduce the performance of the barrier during an impact. National guidelines address some issues related to barrier-mounted sign supports. The AASHTO Roadside Design Guide is the primary national resource that directly addresses the barrier ZOI and identifies some options for reducing exposure of objects located in the zone, including moving the sign support to a location outside the ZOI or modifying the barrier configuration to reduce exposure of the sign support within the ZOI. However, there is limited guidance on how well specific rigid barriers accommodate attached sign supports and limited criteria for placing signs within the ZOI. Some of the data required to establish such guidelines are not available; thus, many barrier-mounted sign details are designed independently by individual agencies, with varying degrees of consideration for impact safety. Forty-six state departments of transportation and five prominent toll road authorities submitted responses to a questionnaire distributed as part of the synthesis study. Results of the survey show that tens of thousands of barrier-mounted sign supports are currently installed in the United States and that transportation agencies use a wide variety of mount- ing details. Many of these signs are located within the barrier ZOI and could present a safety concern if a vehicle crashes into the host barrier in the vicinity of the sign support. This study identifies practices used throughout the nation for mounting sign supports on median barriers and methods used to reduce ZOI exposure. For large overhead sign sup- ports, results show that lateral widening of the barrier cross-section in the vicinity of the supports is the most common technique for reducing ZOI exposure. For small sign sup- ports, relocating or otherwise eliminating the need for a barrier-mounted sign support was reported as the most common technique for addressing ZOI concerns. Transportation agencies can use this review of existing practices to compare their practices with those of others and determine whether they need to revise their standards and policies to improve the overall safety of their divided highway networks. Transportation agencies may benefit from research that aims to define ZOI characteris- tics for common barrier designs to identify which sign-mounting practices perform best. Performance evaluations can provide information on the safety issues associated with existing installations and the effectiveness of available treatments to mitigate ZOI expo- sure. In addition, new techniques for improving impact performance of sign/barrier com-
2 binations need to be evaluated to ensure that sign functionality and driver expectancy are not compromised. Results of these research efforts can help transportation agencies evaluate their current practices and provide a basis for making improvements.
3 information to assist with regulatory compliance, warnings of potential hazards, and navigation guidance. Regardless of the messages they convey, the physical configuration of signs can be separated into two basic categories: (1) over- head signs, which are suspended over the roadway by a can- tilever, span, or bridge support structure; and (2) roadside signs, which are attached to ground-mounted supports adja- cent to roadways. Both types require at least one support anchored to the ground or another fixed object. The size, weight, and configu- ration of a sign support are typically determined by structural analysis of the load it must bear. The geographic region and the size of the sign are usually the primary factors that dic- tate the service loads used to design the required support. For example, larger sign panels require larger and heavier sup- ports. Likewise, signs located in regions that experience high wind speeds require more substantial supports than the same size panels located in regions that have lower wind speeds. AASHTO specifications typically govern the structural design of highway sign supports; however, some agencies supplement or supersede the national specifications to better reflect conditions in their jurisdiction (AASHTO 2009b). Overhead signs are suspended over a roadway by mount- ing them on a grade-separation structure (highway bridge) or by constructing a sign span or cantilever support. Although mounting overhead signs on grade-separation structures is preferred, most overhead signs need to be placed where no suitable highway bridge is available. Currently in the United States, all overhead sign supports are treated as fixed objects and, therefore, must be located outside the roadside clear zone or shielded with an appropriate crashworthy bar- rier (FHWA 2009; AASHTO 2009b, 2011). Figure 1 shows several configurations of overhead sign supports that are typical for divided highways with rigid median barriers. The full span shown in Figure 1a is often preferred, since a median-barrier-mounted support is not required. However, a full-width span is not possible in many locations; in those cases, median-barrier-mounted sign supports are necessary, as shown in Figure 1 b, c, and d. Roadside signs are typically located on the right-hand side of travel lanes, where they are more easily recognized and understood by road users (FHWA 2009). U.S. driv- ers expect to find signs and messages on the right-hand CHAPTER ONE INTRODUCTION BACKGROUND Median barriers are longitudinal barriers used to separate opposing traffic lanes on divided or limited-access high- ways. They are designed to contain and redirect errant vehi- cles that strike them on either side, and their performance requirements are identical to those for roadside barriers (AASHTO 2011). The most effective method of assessing barrier performance is by evaluating results of standard- ized full-scale crash tests. During these tests, the barrier must satisfy a number of requirements related to structural adequacy, occupant risk, and vehicle trajectory (Ross 1993; AASHTO 2009a). Individual barrier designs are typically designated as rigid, semi-rigid, or flexible, depending on the amount of lateral deflection that occurs during impact tests. Semi-rigid and flexible barriers allow some level of lateral displacement as they redirect an impacting vehicle, whereas rigid barriers remain essentially static during and after a design impact event. The type of median barrier used for a particular high- way section depends primarily on the width of the median, although other factorsâsuch as traffic volume and percent- age of truck trafficâmay also be considered. Rigid barriers are more applicable for narrow medians because they are designed to prevent or minimize any encroachment of the barrier or the vehicle into opposing traffic lanes for events up to a specified level of impact severity. Although rigid median barriers are more costly to install initially, they offer the benefit of reduced maintenance and repair costs, making them more desirable for locations with high traffic volume or impact frequency. These factors have triggered an increase in the use of rigid median barriers throughout the United States. Many divided highways have been expanded with additional traf- fic lanes added in the median space, significantly reducing median widths and creating the need for installation of rigid median barriers between the new opposing travel lanes. In other cases, traffic volumes have increased to the point where new or upgraded median barriers are necessary, and rigid types are selected for their low-deflection performance, longevity, and maintenance benefits. Highway signing is an integral component of safe and efficient roadway networks. Signs provide drivers with vital
4 FIGURE 1 Typical overhead sign support configurations.
5 side, as they travel on the right side of the road. However, in some circumstances it is necessary to place signs on the left-hand side. When this occurs on divided highway sections with rigid median barriers, supports for these signs often must be mounted on top of the barrier. Fig- ure 2 shows several configurations for this type of sign installation. Most rigid barriers are relatively narrow, with room for only one or two closely spaced vertical supports. Because of this restriction, a majority of these signs are in the small sign category [panel area less than 50 ft2 (5 m2)] (AASHTO 2009b). Mounting both overhead and small signs on top of rigid median barriers can enable transportation agencies to con- vey important messages to drivers when no other appropriate sign locations are available. The support bases are elevated above the pavement surface, which can provide benefits for maintenance operations such as street sweeping, mowing, FIGURE 2 Typical small barrier-mounted sign supports.
6 and snow removal. Additionally, barrier-mounted supports may be less likely than ground-mounted roadside signs to be struck and damaged by errant vehicles, which reduces repair costs, particularly for nuisance hits. Although these installations provide some benefits, there are safety issues associated with the widespread use of median-barrier-mounted signs, especially the potential for increasing the consequences of vehicle impacts into the bar- rier in the vicinity of a sign support. One concern is that a sign support can diminish barrier performance by creating a snag point that could inhibit the smooth redirection and stability of the vehicle as it is being captured or contained by the rigid barrier. Vehicle snagging during a barrier impact can increase the risk of injury to occupants. An additional safety consideration is that all or part of the barrier-mounted sign structure could become dislodged during an impact event, and these loose components could create a secondary hazard for other motorists. Narrow medians are particularly susceptible to this hazard because of the close proximity of high-speed travel lanes in opposite directions. Barrier research and crash testing have identified poten- tial problems with the safety performance of various barrier designs when objects, such as sign supports, are attached in the vicinity of the impact location (Keller 2003; Caldwell 2011). Many factors and design details of both the barrier system and the sign support affect crash performance of the median struc- ture. Some sign/barrier combinations may perform satisfacto- rily during crash tests, whereas others may be unacceptable, based on standardized evaluation criteria. Although there has been some research and crash testing of specific combinations, limited guidance is available to highway designers, who often must specify safe and adequate systems for their projects. We need to evaluate the state of current practice in order to assess and prioritize needs for improved guidance and research that will advance the quality of future practices. This synthesis study was conducted to address these needs through an information-gathering and -organizing process that presents the available information in a usable reference for future work. SCOPE The synthesis study focuses on critical topics within the wide range of issues and circumstances related to the sub- ject matter. First, the study considers only barrier-mounted sign use in the United States. Information was gathered from state departments of transportation (DOTs) and select toll road authorities whose jurisdictions are entirely within the United States. Although information on practices in other countries is valuable, most of the applicable design and per- formance specifications and guidelines are issued by U.S.- based government authorities. Second, the study considers only permanent barriers and signs. Design guidelines and specifications for high- way features differ for permanent applications and tem- porary or work-zone applications. In general, permanent features are subject to more stringent standards and per- formance requirements because they are in service for substantially longer periods of time and, therefore, have greater exposure to traffic and environmental conditions. This study focuses on permanent barrier-mounted sign installations, which are considered to have a design life of 10 years or more. Third, the study focuses on rigid barriers used in median applications. In permanent median applications, signs are mounted on or attached to vehicle barriers. Roadside barri- ers typically have clear space behind them that can be used for sign supports, so they do not usually need to be mounted directly on top of the barrier. Also, concern about flying debris during an impact is not as critical for roadside barri- ers, as there are no traffic lanes on the other side. Likewise, only rigid barriers are included in the study because they are used in narrow medians, where space limitations are likely to require that any signs be mounted directly on top of the barrier. The vast majority of rigid median barriers currently in use are constructed of reinforced concrete, and designs vary considerably with respect to width, height, and cross- sectional shape. These characteristics affect the barriersâ impact response and level of performance. Differences and similarities in design details and com- ponent properties among agencies are identified for both overhead and small signs mounted on top of or within rigid median barriers. The scope of the study does not include very small, self-supported panels, such as delineators, reflectors, or mile markers less than 0.5 ft2 (0.05 m2) in panel area that do not have a separate support or post component. These lightweight barrier attachments are not considered to be sign structures. On the other hand, barrier-mounted mile markers supported by a vertical post (as shown in Figure 2c) are included in the study. To maintain its focus on the wide variety of applicable sign structures, the study does not include other barrier-mounted highway features, such as luminaire poles, electronic equip- ment poles, railings, fences, and screens. However, many issues addressed in this study apply to those structures as well, and similarities are identified where applicable. STUDY APPROACH The primary objectives of this synthesis study are to locate, assemble, and document available information pertaining to agency practices for use of permanent highway signs mounted on rigid median barriers. Information was gathered using the following methods:
7 â¢ Literature review of state DOT standard plans and specifications to identify current practices in use throughout the United States. â¢ Literature review of national design policies, specifica- tions, and guidelines to assess their adequacy. â¢ Literature review of completed and ongoing research efforts directed at evaluating and solving known prob- lems associated with median-barrier-mounted sign installations. â¢ Review and analysis of responses to a comprehensive survey questionnaire that was prepared and sent to all state DOTs and to select toll road authorities with potentially significant numbers of median-barrier- mounted signs. The questionnaire was published electronically on a spe- cialized website that provides survey services to NCHRP. Invitations to participate in the survey were sent to all voting members of the AASHTO Subcommittee on Design; respon- dents either personally prepared their agencyâs response or assigned it to be completed by appropriate personnel within their agency. The questions and responses are intended to estimate the extent of use of barrier-mounted signs through- out the United States, classify standard design details for common types of installations, and identify practices used to improve performance. The information gathered from all these methods has been evaluated and organized as a reference for highway design practitioners. Chapter two provides a summary of pertinent design criteria that are currently available and in use. Chapter three summarizes the results of the survey questionnaire and identifies common practices among the agencies that participated. Chapter four describes testing and research that have been conducted to define zones of intrusion (ZOIs) and to evaluate impact performance of spe- cific sign/barrier combinations. Chapter five offers conclu- sions regarding the state of practice and recommendations for research to improve existing resources. The report as a whole can be used by transportation agencies to promote successful practices, avoid known problems, and supple- ment their own design policies.