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1 Summary Roadside safety devices are designed to reduce the risk of occupant injuries when a vehicle runs off the road. New devices are evaluated by crash test- ing: the device is installed at a test facility, a vehicle collides with it, and en- gineers assess the consequences of the crash. Guardrails are roadside safety devices installed at locations that do not provide a clear zone in which a vehicle can decelerate without striking an object or encountering unsafe terrain. The end of a length of guardrail must be designed so that it is not a hazard to occupants of a vehicle striking it. Highway agencies install end treatments in a variety of forms intended to absorb energy in a crash and to redirect the vehicle into a safe trajectory. The Transportation Research Board formed a committee with the spon- sorship of the National Cooperative Highway Research Program (NCHRP) to develop a research design for evaluating the in-service performance of guardrail end treatments, determine the data required for the analysis, examine state data systems to determine whether the required data would be available, and identify next steps for carrying out evaluations. The com- mitteeâs conclusions about the objectives of in-service evaluation of end treatments and other roadside safety devices and about current capabilities and evaluation methods for conducting in-service evaluations are summa- rized next, followed by recommendations for proceeding with evaluations.
2 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS CONCLUSIONS Need for In-Service Evaluation In-service evaluation can help to ensure that roadside safety devices effec- tively reduce the risk of injuries and fatalities. Crash testing cannot repro- duce the variety of characteristics of crash dynamics, sites, and installations that affect crash outcomes. In-service data are necessary to determine the frequencies of various crash, installation, and site characteristics so as to determine the crash conditions that should be included in crash tests. In- service evaluation also is necessary to verify that devices in the field per- form as they do in testing. Periodic evaluation is needed because vehicle, traffic, and road characteristics change over time and new device designs frequently come into use. Validation and refinement of crash testing would be appropriate objectives of a nationally coordinated program of evalua- tion research. Highway Agency Readiness for Conducting Evaluations Highway agencies are not prepared to commit resources to systematic in- service evaluation of roadside safety devices. Reasons for not undertaking evaluations are the limitations of data systems, lack of arrangements with police for timely receipt of crash reports, lack of funding and staff, and lack of perceived benefit. Agencies are unlikely to invest in evaluation capabili- ties without evidence that the results can be useful for guiding decisions on selection, maintenance, and replacement of devices. Initial Steps Toward Conduct of Evaluations Trials to demonstrate methods and applications are a necessary first step toward establishing capabilities for in-service evaluations. More informa- tion about the benefits and practicality of routine in-service evaluation is needed before new data collection and analysis programs can be launched. Also, a comprehensive comparative evaluation of the effects on injury risk of alternative device designs and of installation and maintenance practices would have a greater chance of success if it were approached in stages. In the initial stage, studies of modest scale would develop data collection and modeling methods. The initial results would indicate whether the potential benefits justified further research investment.
SUMMARY 3 Evaluation of Roadside Safety Devices Other Than Guardrail End Treatments In-service evaluation is equally justified for all roadside safety devices (in- cluding barriers, crash cushions, end treatments, and support structures). Uncertainty about the reliability of crash testing as an indicator of perfor- mance in service applies equally to all such devices. Data systems used for evaluation are more likely to be cost-effective if they have applications for a range of needs. Methods suitable for evaluating guardrail end treatments also would be applicable for evaluating the performance of other roadside devices. Evaluation Objectives There is a need for the capability to conduct two kinds of in-service evalu- ation of roadside devices, each with distinct objectives: (a) a nationally coordinated evaluation research program to meet common needs of the states and (b) routine in-service evaluation of roadside devices by state highway agencies. Objectives that would be appropriate within the scope of a nationally coordinated program include the following: ⢠Validating and refining crash test procedures to improve the reli- ability of testing as an indicator of the performance of the device in use; ⢠Demonstrating cost-effective methods for routine in-service evalu- ation by a highway agency; and ⢠Evaluating how design, installation, maintenance, and deteriora- tion in use affect performance to help highway agencies define cost-effective practices for selection, inspection, and replacement. Routine in-service evaluation by highway agencies could have at least three applications: ⢠Providing notice that a device type is not performing as expected, ⢠Long-term monitoring to identify cost-effective practices for device selection and maintenance, and ⢠Ensuring that devices are appropriate for their locations and are properly installed and maintained. Because of the lack of highway agency experience in conducting evalua- tions, the practicality, appropriate scale, and utility of such routine evalua- tion activities have yet to be determined.
4 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Evaluation Methods A program of in-service evaluation of roadside safety devices (whether a national program of evaluation research or routine evaluation by a state highway agency) will have three essential components: ⢠An administrative and planning structure that defines evaluation objectives, the scope of evaluation, and responsibilities for evalua- tion and that oversees the application of results; ⢠General-purpose data systems for recording roadway and traffic characteristics, maintenance activities, and crashes that are ad- equate to support the objectives of the evaluations; and ⢠An evaluation methodology that specifies performance measures and methods for computing them. Past activities have laid the groundwork for a methodology, especially a 2003 NCHRP report on evaluation, the 2015 investigations of end treat- ment crashes conducted by a federalâstate task force, and the Federal High- way Administration (FHWA) pilot end treatment evaluation begun in 2016. The experience of these past studies has shown that the principal tech- nical obstacles to evaluation are ⢠Obtaining prompt notification of crashes so that data may be ob- tained at the site, ⢠Obtaining notification of crashes not reported to police, ⢠Obtaining a sample of crashes large enough to reveal infrequent failures and to allow inferences about determinants of severity, and ⢠Ensuring the reliability of data. An evaluation methodology must include procedures for overcoming these obstacles. The past studies will be especially useful models in future evaluations for their definitions of required data elements and data coding methods and for their demonstrations of notification and communication arrangements. The past studies have been less successful at demonstrating statistical meth- ods of controlling for confounding factors (i.e., the influence on crash sever- ity by factors other than the design and condition of the roadside device, such as road and traffic characteristics), methods of selecting scientifically valid samples, and auditing data quality.
SUMMARY 5 RECOMMENDATIONS Validation of Crash Test Procedures The U.S. Department of Transportation (U.S. DOT) and the state depart- ments of transportation should cooperate in undertaking a research pro- gram to validate and refine crash testing of guardrail end treatments and other roadside safety devices through in-service evaluation and simulation modeling. Any evaluation to validate or improve crash testing will require assembling a database of crashes involving the device being evaluated. The database should have the following properties: ⢠A sample of crashes that is representative of the population of crashes, ⢠A sample size that allows the frequency of rare events and of crash characteristics to be estimated, and ⢠Information about the crash and the roadside device that is suf- ficient to compare the circumstances of the crash with the charac- teristics of tests and the outcome with the outcomes of test crashes. The procedures of the FHWA pilot in-service evaluation of end treat- ments should be the model for data collection. The pilot results may in- dicate opportunities for simplifying data collection. The recommended evaluation research program should improve on two aspects of the methods of the FHWA pilot: ⢠The research program should assemble a scientifically valid sample of crashes for analysis, and ⢠The program should apply quantitative performance measures, including the severity distribution of crash outcomes. The analysis for validating crash test procedures should have two com- ponents: comparison of actual crash circumstances with circumstances in the tests and comparison of the outcomes of actual crashes that match test circumstances with test outcomes. The evaluation should estimate the frequency of each defined category of crash conditions (e.g., crash conditions that correspond to prescribed test conditions) and the increase in injury risk associated with each condition. If crashes outside the range of test conditions are found to cause significant numbers of casualties, or if outcomes of actual crashes that match test circumstances differ from test outcomes, then the research should identify changes in testing to more accurately predict the performance of devices.
6 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Assessment of Simulation Modeling The crash test validation research program should include an assessment of the usefulness of simulation models in conjunction with crash testing to certify new device designs. In-service evaluation findings and crash test results can support the specification and validation of simulation models of vehicle collisions with roadside safety devices. The research should examine the application of models to evaluate (a) device performance in crash cir- cumstances that would be impractical to test physically and (b) the effects of site details such as placement of the device, slopes, and soil conditions. Demonstration of Evaluation Methods for Routine Highway Agency Use The state departments of transportation, or the U.S. DOT and states act- ing cooperatively, should conduct a demonstration of methods suitable for highway agencies to use for routine in-service evaluation of roadside devices. The demonstration should test methods and determine whether evaluation can be useful for improving the safety and cost-effectiveness of highway programs. The results would be used to revise guidance on evalua- tion and to develop training materials. The nationally coordinated program would provide technical support and coordination. Participating agencies would independently test methods with their own resources and assess the utility of results. Participants would be agencies with maintenance manage- ment systems, and data and analysis processes would be integrated with these existing systems. Participating agencies would be encouraged to include trials of new technology for data capture and trials of contractual innovations that give the manufacturer or installer responsibility for providing inventory and performance data. The demonstration should include an analysis of agen- ciesâ costs and staff time requirements for data collection and evaluation. Benefits should be characterized in terms of the impact of information from the evaluation on agency decisions and practices. Evaluating Effects of Design, Installation, and Maintenance Practices on Performance The U.S. DOT and the state departments of transportation should begin exploratory data analysis toward the development of a statistical modeling approach to measure the effects of device design, installation, maintenance, and site characteristics on the performance of guardrail end treatments and other roadside safety devices. The exploratory analysis should begin with the data collected in the crash test validation study recommended above. The results could support highway agency decisions regarding the selection
SUMMARY 7 of device types to be installed at particular locations and the priorities for maintenance and replacement of the devices. The recommended explor- atory analysis would provide a basis for deciding the appropriate scale and direction of future research on development and application of crash severity models. Organization of the Nationally Coordinated Evaluation Research Program The U.S. DOT and the states should consider at least two alternative orga- nizational forms for planning and oversight of the nationally coordinated evaluation research program: ⢠Extension of the charge and term of the American Association of State Highway and Transportation OfficialsâFederal Highway Ad- ministration (AASHTO-FHWA) Task Force on Guardrail Terminal Crash Analysis, as a means of coordinating state and federal inter- ests and resources and ⢠An AASHTO-led effort conducted through the National Coopera- tive Highway Research Program. The entity overseeing the evaluation program should first develop a plan that defines the objectives of evaluations (i.e., how the results will be ap- plied in the management of the highway system), scope (e.g., which devices are to be evaluated), funding needs, and schedules. The entity also will be responsible for obtaining cooperation of the federal, state, and local agen- cies and offices that would be involved and for monitoring the conduct of evaluations and applications of results.
