CRASH ATTRIBUTES THAT INFLUENCE THE SEVERITY OF ROLLOVER CRASHES Kennerly H. Digges

This paper examines NASS/CDS 1995 to 2001 in order to evaluate rollover crash severity metrics and to recommend additional data elements to assist in determining rollover severity. For belted occupants and unbelted ejected occupants in single vehicle crashes, the number of roof impacts is an appropriate severity indicator. For ejected occupants, the ejection risk generally increases with the number of quarter-turns. However, number of roof impacts provides a more uniform relationship between crash severity and injury risk. For the population of crashes with another vehicle prior to rollover, the injury risks are higher than for single vehicle crashes. Impacts with a fixed object such as a tree or wall prior to rollover also carry higher injury risks. Consequently, additional severity measures such as the delta-V for the pre-rollover crash event are required in addition to the rollover severity measures. For non-ejected unbelted occupants in single vehicle crashes, the most frequent and most injurious category was one quarter-turn. An examination of cases with one quarter-turn showed that the majority of the injuries were in rollovers that were stopped by impacts with trees and poles. In most of these cases, the point of impact was the roof. Additional data elements to permit easy identification of rollover cases that also involve planar crashes with fixed objects during the rollover would be desirable. Crash severity metrics associated with planar crashes may be applicable to these cases. INTRODUCTION Rollover crashes continue to be a growing source of motor vehicle injuries and deaths in the United States. One of the difficulties in analyzing rollover crashes and designing countermeasures is the lack of a standard measure of crash severity. Rollover severity factors equivalent to delta-V, used in planar crashes, are not currently available in the crash data files. A number of factors are currently recorded that might be related to crash severity. These include: pre-crash speed (estimate), number of quarter-turns of roll, extent of vehicle damage, single or multivehicle event, and rollover initiation type. In earlier studies of accident data, Malliaris found that precrash velocity was the most influential rollover crash severity parameter [Malliaris, 1991]. In a companion paper, Digges applied computer modeling to rollover crashes and found that increases in vehicle rotation rate and vertical velocity also increased rollover severity [Digges, 1991]. None of these suggested parameters (pre-crash velocity, vertical velocity, and rotation rate) are readily available from after-the-fact crash investigations. However, in the future, onboard crash recorders may make it practical to collect these added parameters in real time and store the results. The purpose of this paper is to analyze recent NASS/CDS rollover crashes to determine rollover crash severity variables that can be measured or computed using currently available crash investigation methods. The suitability of attributes that can be based on data elements currently documented in NASS/CDS is the primary focus of this analysis. Crash severity is an important factor in calculating the benefits of a countermeasure. It is desirable to have a crash severity measurement that can be related to the rate of injury. When a safety feature is introduced that reduces the risk of injury for a range of crash severities, the injury reduction for a fleet of vehicles can be calculated. For the analysis to follow, the recommended crash severity parameters are those that predict increasing injury rate with increasing crash severity. The injury rate is measured