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From page 55... ... 3 Nationally Coordinated Evaluation Research In Chapter 1 it was noted that defining the objectives of the evaluation in terms of the intended applications of the evaluation results is the necessary first step in developing an evaluation method. The evaluation can then be designed to produce the results required for those applications. Read the entire page →
From page 56... ... 56 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS subsequent three sections outline methods for each of the three candidate objectives. The methods proposed would be applicable to evaluation of guardrail end treatments, and could be adapted for evaluations of other roadside safety features. Read the entire page →
From page 57... ... NATIONALLY COORDINATED EVALUATION RESEARCH 57 SOURCES FOR EVALUATION PROCEDURES The experience of past evaluations described in Chapter 2 indicates that the major difficulties in conducting a prospective in-service evaluation of a roadside safety feature include the following: • Obtaining notification of relevant crashes quickly enough that data on the performance of the device and other circumstances of the crash may be obtained at the site; • Obtaining information on crashes not reported to police, which is needed if the complete severity distribution of crashes is the measure of performance; • Obtaining a sufficiently large sample of crashes to allow for reliable estimation of the effects of the device type and other factors on severity (or on crash frequency, in the case of an evaluation of a road feature intended to reduce crash risk, such as rumble strips) ; and • Ensuring the consistency and reliability of data, given that data collection may involve multiple participants (police, maintenance workers, crash investigators) Read the entire page →
From page 58... ... 58 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS and the procedure chosen for a particular evaluation must be appropriate for the objective of the evaluation, but the past activities provide a groundwork for evaluation methodology and are applicable to the evaluations proposed in this chapter. VALIDATING CRASH TEST PROCEDURES Objective and Applications The 2015 AASHTO-FHWA Task Force investigation demonstrated that end treatment crashes occur that have potentially severe consequences and involve crash dynamics that are not represented in current and past crash testing protocols. Read the entire page →
From page 59... ... NATIONALLY COORDINATED EVALUATION RESEARCH 59 Data Collection The basic requirement for an evaluation to validate or improve crash testing of a roadside safety device is a database of crashes involving the device. The database should have the following properties: • The sample of crashes is representative of the population of crashes; • The sample is large enough that rare events can be observed and the frequency of crash characteristics can be estimated; and • Information about the crash scenario, crash site environment, and pre- and postcrash conditions of the roadside device are sufficient to compare the circumstances of the crash with the characteristics of tests and the outcomes with the outcomes of test crashes. Read the entire page →
From page 60... ... 60 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS terms of measurements of crash characteristics and device response that are recorded in the crash database. • Case samples should be chosen by statistically valid methods. Read the entire page →
From page 61... ... NATIONALLY COORDINATED EVALUATION RESEARCH 61 • Hardware installation/maintenance/repair [e.g., installation not complying with manufacturer's drawings] Read the entire page →
From page 62... ... 62 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS in the AASHTO-FHWA Task Force report described above) of a comparison of the circumstances of actual crashes with crash test impact conditions. Read the entire page →
From page 63... ... NATIONALLY COORDINATED EVALUATION RESEARCH 63 Information on crash outcomes also is necessary to determine the range of impact conditions that crash tests should cover. The 1986 study concluded that the crash tests were stringent because only small percentages of crashes exceeded both the test criteria. Read the entire page →
From page 64... ... 64 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS • If in-service data show that a crash scenario not similar to any of the standard crash tests occurs with some frequency but too few cases of the scenario are observed to reliably estimate the likelihood of a severe outcome, simulation can be used to predict severity. • If poor performance of a roadside device is observed in an inservice evaluation, simulation can help to determine the contributions of device design and site characteristics (e.g., slopes, ditches, soils, installation errors) Read the entire page →
From page 65... ... NATIONALLY COORDINATED EVALUATION RESEARCH 65 safety devices. Models today are validated through a comparison of model predictions with crash test results. Read the entire page →
From page 66... ... 66 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS crash tests cannot be replaced by simulation modeling (AASHTO 2009, 205) Read the entire page →
From page 67... ... NATIONALLY COORDINATED EVALUATION RESEARCH 67 Each agency would prepare a plan for data collection and analysis and for application of evaluation results in maintenance, construction, and safety management decisions. Data collection, database structures, and analysis procedures would be integrated as far as possible with the existing maintenance management and safety management systems. Read the entire page →
From page 68... ... 68 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS design, installation, maintenance, and site characteristics on the performance of guardrail end treatments and other roadside safety devices, so as to provide guidance to highway agencies on the use of these devices. The results would support highway agency decisions in three areas: • Selection of device types to be installed at particular locations. Read the entire page →
From page 69... ... NATIONALLY COORDINATED EVALUATION RESEARCH 69 formance limitations (Joint AASHTO-FHWA Task Force on Guardrail Terminal Crash Analysis 2015, 5) Read the entire page →
From page 70... ... 70 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS considered only device type and location characteristics would not require postcrash device condition data. The analysis would quantitatively estimate the simultaneous effects of the device type, installation and maintenance features, and site features on the distribution of crash severity. Read the entire page →
From page 71... ... NATIONALLY COORDINATED EVALUATION RESEARCH 71 solely to guardrail end treatments, which are involved in only a small share of serious crashes, would be difficult to justify. A more cost-effective activity would be to identify in-service evaluation needs covering guardrails together with guardrail end treatments or all MASH devices. Read the entire page →
From page 72... ... 72 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS The ideal test validation study would observe a sufficient number of crashes in each category to estimate the frequency of occurrence of each of the categories and to compare in-service crash outcome (i.e., the severity distribution) in each category with the outcome of the corresponding crash test. Read the entire page →
From page 73... ... NATIONALLY COORDINATED EVALUATION RESEARCH 73 crashes, it would be necessary to observe several hundred crashes involving the device. If the device's true failure rate were 1 percent, then in a sample of 400 crashes, the probability of observing two or more failures would be 90 percent; therefore, if one or no failures were observed, the agency could be reasonably confident that the failure rate was less than 1 percent. Read the entire page →
From page 74... ... 74 T A B L E 3 -1 G ua rd ra il E nd T re at m en t C ol lis io n R at es R ep or te d in I nSe rv ic e E va lu at io ns R oa d C la ss Se gm en t L en gt h (m ile s) Read the entire page →
From page 75... ... NATIONALLY COORDINATED EVALUATION RESEARCH 75 the number found in maintenance or police records (Ray and Hopp 2000, 47) Read the entire page →
From page 76... ... 76 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS NHTSA's NASS-CDS provides an indication of costs. The cost of operating the NASS-CDS in 2013 was $3,600 per crash case (GAO 2015, 22) Read the entire page →

From page 55...

... 3 Nationally Coordinated Evaluation Research In Chapter 1 it was noted that defining the objectives of the evaluation in terms of the intended applications of the evaluation results is the necessary first step in developing an evaluation method. The evaluation can then be designed to produce the results required for those applications.