In-Service Performance Evaluation of Guardrail End Treatments (2018) / Chapter Skim
Currently Skimming:
Pages 28-54
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 28... ...
2 Methods of Measuring Performance Chapter 1 identified three components essential to a program of in-service performance evaluation: the administrative and planning framework that determines the scope and objectives of evaluation, responsibilities for evaluation, and the applications of results; data systems, which constitute the infrastructure of evaluation; and evaluation methods, that is, specifications for analysis of crash and roadway data to measure performance. This chapter presents examples of evaluation methods.
|
From page 29... ...
METHODS OF MEASURING PERFORMANCE 29 standard practice in research on the safety effects of roadway and vehicle design features and the benefits of safety interventions. In the examples of methods of evaluation of guardrail end treatments given below, the primary measure of performance usually is the distribution of the severity of crash outcomes (for example, the fraction of all collisions with an end treatment that result in a severe injury or fatality)
|
From page 30... ...
30 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Box 2-1 Prospective Evaluation Example: Washington State Evaluation of Guardrail End Treatment The Washington State Department of Transportation undertook an in-service evaluation of guardrail end treatments in use in the state to help determine state policy regarding replacement of terminals of older designs (Igharo et al.
|
From page 31... ...
METHODS OF MEASURING PERFORMANCE 31 Type Disabling Injury Other Injury Property Damage Only BCT 3 4 11 SRT 1 2 6 A test of statistical significance of the difference in severity distribution between the two types shows that the hypothesis (that the probability of crash outcome is independent of end treatment type) cannot be rejected.
|
From page 32... ...
32 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS • The comparison of end treatment types with respect to crash severity does not adequately take into account factors other than the design of the devices that may affect severity. Road conditions and crash characteristics varied greatly: roads included Interstates and minor low-volume roads; ramps were the locations of 3 percent of end treatment installations but 16 of the 30 crashes observed.
|
From page 33... ...
METHODS OF MEASURING PERFORMANCE 33 Step 1. Safety Performance Hypotheses The essential first step of the evaluation is to define the hypotheses to be tested to ensure that the necessary data are obtained and that the planned sample size will be adequate and to avoid effort in collecting inessential information.
|
From page 34... ...
34 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS required to allow the hypotheses to be tested. The number of observations obtained will depend on the extent of the roads included in the study, the density of the roadside features of interest, traffic volume, and the frequency of crashes per vehicle passing the features.
|
From page 35... ...
METHODS OF MEASURING PERFORMANCE 35 Obtaining prompt, reliable reports of relevant crashes has been a difficult organizational challenge in past in-service evaluations. The procedure for a prospective in-service evaluation generally will call for inspection of crash sites as soon as possible after the event.
|
From page 36... ...
36 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS of the evaluation. Box 2-3 shows the crash data elements that the Federal Highway Administration (FHWA)
|
From page 37... ...
METHODS OF MEASURING PERFORMANCE 37 Some past evaluations have restricted crashes included in the analysis to those in which collision with the roadside device to be evaluated was the most harmful event (as judged by the police officer completing the crash report form) , reasoning that, in other crashes involving the device, the outcome severity was determined by events other than collision with the device.
|
From page 38... ...
38 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS • Information about damage to and repair of the devices involved in all crashes is needed if the evaluation is to compare the costeffectiveness of alternative types of devices. More experience with the conduct of in-service evaluations will be necessary before the utility of data on non-police-reported crashes can be assessed.
|
From page 39... ...
METHODS OF MEASURING PERFORMANCE 39 Step 4. Assembly of Roadway Data Possible uses of information on roadway and roadside characteristics in the study area and at crash sites include the following: • Data on the locations and design types of the roadside features to be evaluated are necessary for the selection of the study area.
|
From page 40... ...
40 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS to mitigate crash outcomes. Some evaluations have used simplified forms of severity distribution for comparing the safety performance of alternative types of roadside features, including • The rate of severe crashes (crashes resulting in fatal or incapacitating injuries)
|
From page 41... ...
METHODS OF MEASURING PERFORMANCE 41 • Assuming that the locations of a roadway safety device type are not correlated with roadway and traffic characteristics that affect crash risk (i.e., that a particular device type is not more likely to be found at a location with a high crash risk than at a low-risk location) , • Estimating a multivariate model of crash severity, • Borrowing an existing model of crash frequency or severity and examining whether prediction error correlates with roadside device design type,2 and • Using a case-control study design.
|
From page 42... ...
42 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Box 2-4 Retrospective Evaluation Example: Factors Affecting Injury Risk in Crashes with Guardrails and Guardrail End Terminals In a study of injury risk in frontal crashes with guardrail and guardrail end terminals, Johnson and Gabler used historical crash data compiled by the U.S. Department of Transportation to identify the factors influencing the severity of crashes with guardrails and guardrail terminals (Johnson and Gabler 2014)
|
From page 43... ...
METHODS OF MEASURING PERFORMANCE 43 ated by the impact, was the most harmful event. The crashes that were identified as meeting these criteria involved a total of 711 vehicles.
|
From page 44... ...
44 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Box 2-5 Descriptive Evaluation Example: Trial Installation: Evaluation of Wisconsin ET-2000 The Wisconsin Department of Transportation undertook an evaluation of a particular guardrail end treatment design (the ET-2000) (Bischoff and Battaglia 2007)
|
From page 45... ...
METHODS OF MEASURING PERFORMANCE 45 Analysis and Conclusions The evaluators concluded that The ET-2000 systems installed as part of this research study have performed well. Vehicles that collided with the end terminals were safely brought to a stop, and very few injuries were reported.
|
From page 46... ...
46 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Box 2-6 Case-Control Evaluation Examples Insurance Institute for Highway Safety Study of Effect of Truck Configuration on Crash Rates The Insurance Institute for Highway Safety's study to observe the effect of tractor trailer configuration on crash rates provides an illustration of the case-control method (Braver et al.
|
From page 47... ...
METHODS OF MEASURING PERFORMANCE 47 that is, (102/1,931)
|
From page 48... ...
48 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Analysis The data were as follows: Treatment Cases (ET terminals involved in severe crashes) Controls (all ET terminals in 10-mile road segment prior to crash site)
|
From page 49... ...
METHODS OF MEASURING PERFORMANCE 49 with guardrails that used FHWA's Highway Safety Information System (a multistate database of crash, roadway, and traffic data [FHWA 2012]
|
From page 50... ...
50 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS 2. Identification of cases.
|
From page 51... ...
METHODS OF MEASURING PERFORMANCE 51 DESCRIPTIVE EVALUATIONS In a descriptive evaluation, the subject roadside feature is regularly observed for a period of time. At the end of the trial period, the evaluator judges whether the feature is performing as intended and whether a need is indicated for any changes in design or in installation or maintenance practices.
|
From page 52... ...
52 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Statistical sampling principles are relevant to the plan of a descriptive evaluation, even if the design does not involve using statistical methods for hypothesis testing. First, the set of cases chosen for examination (e.g., collisions or device installations)
|
From page 53... ...
METHODS OF MEASURING PERFORMANCE 53 Braver, E
|
From page 54... ...
54 PERFORMANCE EVALUATION OF GUARDRAIL END TREATMENTS Ross, H
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.