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 concerns. 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 during 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 mounting 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 supports, 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 supports, 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 characteristics 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 exposure. In addition, new techniques for improving impact performance of sign/barrier combinations 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.