An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance Special Report 265 (2002) / Chapter Skim
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Chapter 5: Summary, Major Findings, and Recommendations for a Future Approach
Chapter 5: Summary, Major Findings, and Recommendations for a Future Approach
Pages 84-95
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From page 84... ...
, defined as its track width, T, divided by twice its center of gravity height, H The congressional mandate for this study requested • A determination of "whether the static stability factor is a scientifically valid measurement that presents practical, useful information to the public"; and • A comparison of "the static stability factor test versus a test with rollover metrics based on dynamic driving conditions that may induce rollover events." The committee's findings regarding SSF and NHTSA's star ratings for rollover resistance are presented below, following some brief remarks designed to place this study in the broader context of road safety and automobile crashes in general.
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From page 85... ...
Design changes that result in a higher rating for one vehicle feature, such as rollover resistance, should not be achieved at the expense of introducing other vehicle attributes that actually make the vehicle less safe overall. The many and complex trade-offs inherent in the vehicle design process make it particularly challenging to achieve overall vehicle safety improvements.
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From page 86... ...
Current understanding of vehicle dynamics indicates that, following initiation of such a rollover sequence, vehicle behavior as the driver loses control is determined by many vehicle design parameters that affect handling (e.g., steering response, brake and suspension characteristics, track width, tire characteristics) ,2 as well as by driver control inputs (steering, throttle, braking)
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From page 87... ...
Therefore, when known risk factors such as young male driver, driver drinking, excessive speed, and driving in a rural area are taken into account, the effect of SSF on the occurrence of rollover is statistically significant. The rollover curves for low-risk scenarios also show a decrease in rollover probability For the purposes of NHTSA's analyses of crash data, rollover risk is defined as the probability of rollover 3 in the event of a single-vehicle crash.
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From page 88... ...
Static Measures and Dynamic Testing As part of its charge, the committee was asked to compare "the static stability factor test versus a test with rollover metrics based on dynamic driving conditions that may induce rollover events." SSF provides important information on a vehicle's rollover propensity in the final phase of a crash. A vehicle's crash-avoidance characteristics are determined by more than 20 vehicle design parameters -- including track width, T, and center of gravity height, H -- that affect handling.
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From page 89... ...
An exponential model is seldom used for analyzing binary data; regression analysis using a logit model is a more appropriate method. In response to a comment to this effect from the Alliance of Automobile Manufacturers, NHTSA recalculated the rollover curve using logistic regression, and found the rollover curves based on the exponential and logit models to be similar (Federal Register 2001a)
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From page 90... ...
This is a coarse approximation that results in a substantial loss of information, particularly at lower SSF values, where the rollover curve is relatively steep. The choice of breakpoints for the rating system does not exploit the richness of the available crash data, and as a result the system is not as helpful as it could be for consumers.
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From page 91... ...
For the analysis of rollover crash data, this model is more appropriate than an exponential model. Finding 6: The approximation of the average rollover curve with five discrete levels -- corresponding to the five rating categories -- is coarse and does not adequately convey the information provided by the available crash data, particularly at lower SSF values, where the rollover curve is relatively steep.
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From page 92... ...
The answers to frequently asked questions about the ratings note that even a fivestar vehicle is not immune from rollover and also warn consumers that any load placed on the roof will be above the center of gravity of the vehicle, thereby increasing the likelihood of rolling over. The list of ratings indicates which vehicles are equipped with electronic stability control, and the accompanying text notes that this feature "may reduce the likelihood of a single vehicle crash, and thus, the risk of subsequent rollover." One of the objectives of dynamic testing is to assess the effectiveness of electronic stability control systems in helping a driver avoid conditions leading to rollover.9 The rollover resistance ratings for individual vehicles represent an attempt to provide relevant information about a complex risk in a concise summary measure.
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From page 93... ...
• The agency has not undertaken empirical studies to evaluate consumers' use of the rollover resistance rating system in making vehicle safety judgments or purchase decisions. In summary, the committee found that NHTSA's star ratings for rollover resistance are likely to be of limited use in presenting practical information to the public because: • There were shortcomings in the statistical methodology used to derive the average rollover curve.
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From page 94... ...
Similarly, the development of consumer information based on static measures and dynamic tests appears feasible, particularly if NHTSA takes advantage of recommended development practices and proven techniques for communicating risk-based information to consumers. Despite the absence of technical barriers to providing more comprehensive consumer information on rollover, the protracted history of NHTSA's rulemaking initiatives on rollover (see Chapter 1)
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From page 95... ...
2001b. Consumer Information Regulations; Federal Motor Vehicle Safety Standards; Rollover Prevention; Request for Comments.
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