The National Academies

NCHRP 22-30 [Completed]

In-Service Performance Evaluation of W-beam End Terminals

  Project Data
Funds: $836,000
Research Agency: TRB Studies and Special Programs Division
Comments: Completed - published as TRB Special Report 323


Crashworthy end terminals are installed at the ends of W-beam guardrail to develop the strength of the barrier system and to provide crash protection to occupants of vehicles that impact the end of barrier installations. Full-scale crash tests for end terminals are based on impact performance criteria as provided in the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH). The Federal Highway Administration (FHWA) procedures require that end terminals installed on federally funded projects must meet MASH or the National Cooperative Highway Research Program (NCHRP) Report 350 crash test criteria for eligible use on the National Highway System (NHS). The AASHTO Roadside Design Guide (RDG) provides guidance on the selection and installation of W-beam guardrail end terminals that are based on systems that comply with MASH or with the earlier NCHRP Report 350 criteria. According to MASH, an “In-Service Performance Evaluation” (ISPE) remains a follow-up to the crash test experiments, which partially assess the efficacy of an end terminal. More thorough and in-depth knowledge of end terminals are essential to the validation of their implementation. NCHRP Report 490: In-Service Performance Evaluation of Traffic Barriers, published in 2003, provides insight to procedures and guidelines for performing in-service evaluations that include the in-service performance evaluation of end terminals. Although the roadside safety community has agreed about the importance of in-service evaluations and procedures in existence, few in-service performance evaluations have been completed. Simply relying on full-scale crash tests to assess the long-term performance of end terminals may overlook important aspects of crash safety as well as other aspects of device performance. Crash tests that are conducted under specific standardized and idealized conditions do not consider the full range of vehicle impacts that are expected to occur in the real world—including such factors as vehicle weight, type, and configuration, impact speed and angle, and vehicle orientation. They do not consider the wide range of environmental conditions to which devices are exposed, such as snow and ice, saturated soil conditions, frozen ground, and extreme temperature ranges. The effects of these conditions are generally unknown, although it is not unreasonable to expect that they may affect device performance. Other factors, such as end terminal maintenance, repair of minor impacts, and variations in device installations can affect their true real world performance. End terminals that perform acceptably during full-scale crash tests may not provide the level of protection expected over the range of these conditions that may be encountered over the life of the end terminal. Evaluating end terminal performance in the field over the life of the device is the most effective way to judge the long-term effectiveness of the hardware. Currently, the majority of states are raising their guardrail heights to the new 31-inch guardrail height requirements while using W-beam end terminals to protect the ends of new installations. Relatively small numbers of newer devices have been successfully crash tested to MASH crash test criteria. While many more end terminals have been successfully crash tested to NCHRP Report 350 crash test criteria, it is expected that not all of these earlier devices will meet the newer MASH criteria. The limited highway resources of many states make it imperative that safety features installed on new construction projects or upgraded on existing highway projects, must be accomplished in the most cost-effective manner possible. Projects funded to replace devices that perform acceptably, while leaving poor-performing devices in service at other locations, is not appropriate. Research is needed to more thoroughly understand the real world performance of W-beam guardrail end terminals through the evaluation of the in-service safety performance of the most common designs currently in service on the nation’s highway system.


The objective of this research is to evaluate the in-service crash performance of a select number of common W-beam guardrail end terminals currently installed throughout the United States. For each common end terminal, the evaluation should include:
  1. Crash performance in terms of vehicle occupant risk.
  2. The sensitivity to varying effects such as environmental conditions, site characteristics, and impact conditions.
  3. The degree of sensitivity to improper installation, maintenance and repair.
  4. The report is intended for the use of transportation agency officials. The research should build upon previous work, provide guidance, and address liability issues for transportation agencies and management professionals.
STATUS: Terminated; transferred under TRB Studies and Special Programs Division; completed as TRB Special Report 323. 

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