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Roof Crush: An Overview of Defect and Injury Mechanisms
Roof structures on many current and past production vehicles fail to withstand the forces of a rollover collision. Structures that fail commonly do so at the roof pillars or the front or rear roof header causing intrusion into the occupant compartment. This compromise of the occupant survival space often results in injuries to the head and spine. Rollover collisions do not generate the high levels of deceleration seen in frontal, side, or rear impacts. In a rollover, energy is dissipated during the rollover a longer period of time without a high level of deceleration being transferred to the vehicle and its occupants. When intrusion does occur, the failed structures frequently lack proper design and construction. Failed designs often lack box section construction or reinforcements in the form of metal or structural foam. Poor manufacturing quality can also lead to roof structure failure.
Most manufacturers design vehicles to meet U.S. Federal Motor Vehicle Safety Standard (FMVSS) 216. The static requirements of this test are widely criticized as a method of determining roof strength during real-world rollover collisions. FMVSS 216 is not representative of the complicated loading applied to a vehicle during a rollover and has been consistently noted as a poor predictor of roof crush in real-world rollover crashes. Studies also show that roof strength, as measured by FMVSS 216, does not correlate with injury in real-world rollover crashes. An alternative test that can be used to meet compliance with U.S. standards calls for a dynamic rollover test; however, most manufacturers choose the static test because it is much less stringent. Only a few manufacturers regularly perform dynamic rollover tests of their vehicles to determine how well occupants will be protected in a rollover collision.
Data shows that roof crush in real-world rollover crashes correlates with increased injury. Head and fracture/dislocation injuries to the neck are common in vehicles with reduced occupant space due to intrusion. Typical injuries from roof crush include a flexion dislocation and/or fracture of the lower cervical spine (C5/C6/C7). This injury is caused by interaction of the occupant's head with the roof. Specifically, the head is pushed down and forward such that the chin is moved toward the chest resulting in flexion of the lower cervical spine well beyond the normal range of motion. This type of loading can also cause a compression fracture of the lower cervical spine. Both of these types of injuries can result in quadriplegia. Depending on the roof crush configuration and occupant kinematics, thoracic spinal injuries and severe brain injuries also occur in these types of incidents. Roof crush injuries can occur with very small amounts of intrusion in vehicles with little initial headroom.
National Highway Traffic Safety Administration (NHTSA) findings dating back nearly 20 years have shown that accident statistics demonstrate that the degree of roof intrusion is highly associated with occupant injury severity. Agency studies have suggested upgrading roof structures and/or improving padding on roof interiors to mitigate roof crush and injuries in rollovers. However, little effort has been made to improve regulations. Currently, NHTSA has requested public comment on changes to procedures for testing the strength of vehicle roof structures under FMVSS 216 -- Roof Crush Resistance. In its request for comments, the agency released findings from its long-awaited analysis of rollover crash data which again found a correlation between roof crush and injury contrary to industry claims that this correlation does not exist.
(CURRENT NHTSA ACTIVITY, see above paragraph)
Government Begins Rulemaking To Improve Roof Strength
The NHTSA’s recent request for comments on changes to FMVSS 216, Roof Crush Resistance marks the beginning of what will likely be a significant battle to improve the antiquated regulation. The current roof crush standard, which became effective in 1973 as a "temporary" requirement, has been modified only slightly since it was promulgated and is widely viewed as having no real-world relevance.
In support of upgrading the test procedures, a recent NHTSA study revealed that approximately 26,376 vehicle occupants sustain serious or fatal injury due to rollover each year and over half of the occupants sustaining injury with the occurrence of roof intrusion were belted. The study concluded that roof crush intrusion is estimated to occur and potentially contribute to serious or fatal occupant injury in about 26 percent of rollover crashes.
In response to NHTSA's request for comments, auto manufacturers argue that a real-world injury reduction will not occur as a result of additional roof strength requirements and that the current standard is adequate. Manufacturers claim that crash data demonstrates that vehicles meeting or exceeding FMVSS 216 are effective in reducing the risk of serious injury to belted occupants and the static test specified in the regulation are the best method to evaluate overall roof strength. The static test requirement simply requires the vehicle roof to support a platen placed on the roof near the A-pillars; however, the test is not representative of the complicated loading applied to a vehicle during a rollover.
Vehicle manufacturers take the position that roof crush is not related to injury and use the "Malibu" test series as validation of this premise. These tests funded by General Motors (GM) in the 1980s compared production vehicles to vehicles with roll cages in rollover tests with restrained and unrestrained dummies. The main conclusion was that roof crush is not the cause of head, neck and spine injuries. The Malibu studies have been criticized because rather than comparing the actual force data from head impacts, they counted the number of head impacts in which force levels exceeded a preset level. As a result, the majority of dummy head contacts in both production and roll caged vehicles exceeded this force level making it appear there was no difference between them. Examination of the actual force level data shows that the average head impact forces were higher in production vehicles in which the roof crushed compared to the modified vehicles that sustained minimal roof damage.
Despite the criticisms and conflicting findings, the manufacturers continue to argue that increases in roof crush resistance above the current FMVSS 216 are unlikely to reduce the neck injury potential of rollovers. They also argue that occupant injury in rollover results from ejection or occupant impact with the vehicle interior, therefore, increased restraint usage would be more beneficial. The deadline for comments ended in December, 2001. NHTSA has not set a date to issue a formal proposal for the upgrade.
Manufacturers' Defense of Roof Crush Cases
Manufacturers have historically taken the position that there is no relationship between roof crush and injury in rollover crashes. Many technical papers published by the industry and its defense experts conclude that roof crush is a factor of increased accident severity and that injury severity increases with accident severity; therefore roof crush is not a direct cause of the injuries. Although they take this position, it has been proven, and accepted by the automotive safety community that on average, deceleration rates in rollover collisions are low and the rollover collision sequence tends to be fairly long. The longer the distance and time the vehicle rolls the lower the deceleration levels experienced by the vehicle and its occupants.
In the 1980s, in an effort to combat the theory that roof crush causes injuries, GM funded a series of restrained and unrestrained dynamic rollover tests. They compared production vehicles to vehicles with protective roll cages. These tests are commonly referred to as the Malibu Series (Malibu I unrestrained tests, Malibu II restrained tests). The primary conclusion from the studies was that roof crush does not cause head, neck, and spine injuries--these injuries allegedly result from the occupant diving into the roof as it hits the ground. Since these studies were published, the findings have been extensively debated and critiqued within the scientific community. Despite valid critiques, the defense in many roof crush cases is based on these studies.
When the data from Malibu are closely scrutinized, the results show that increased roof strength does indeed reduce the potential for injuries to the head and neck and that the conclusions of the tests were flawed because of the use of an unfounded injury criterion. A measurement called the "Potentially Injurious Impact" (PII) was used to compare the dummy responses in vehicles with roll cages to the production vehicles. PII was defined as any impact causing an axial neck load of 2,000 N or higher. The authors counted the number of PIIs in both production and roll caged vehicles to determine if there was any difference for the different roof structures.
This study has been criticized for using the PII criterion rather than a criterion that measures potential for critical injury. It is argued that the authors purposefully set the PII threshold low so that most of the neck loads were above this threshold and thus indistinguishable from each other. Critics state that this is why the authors were able to conclude that there was no difference in occupant injury when the roof crushed. When the data are analyzed with a higher PII, results indicate a reduction in the potential for injury in the roll caged roofs.
An examination of Malibu shows that, overall, the dummies in the roll cage-equipped vehicles had a lower number of potentially injurious impacts and a lower average neck load than the dummies in the production vehicles. However, the PIIs were typically much higher (3000 to 6000 range) than the arbitrary 2000 threshold. Therefore it was claimed that this demonstrated that there was no difference between the production and roll cage-equipped vehicles.
Importantly, while auto manufacturers and their experts stand by the Malibu, buried within their own conclusions are admissions that their data show severe roof crush does correlate with increased injury severity, and the absence of roof deformation may benefit belted occupants if they do not contact the roof. Nevertheless, the "diving" defense has been effective with juries, and most cases that go to trial have ended in defense verdicts.
Recent Settlements and Verdicts
On May 2, 2002, an Alabama jury returned a $122 million verdict against General Motors. Jeffrey Jernigan, age 12, and his 17 year old brother Nickolas, were traveling at approximately 50 miles per hour when their vehicle, a 1993 Oldsmobile Delta 88, collided head-on with another car. The force of the crash tore away about a third of the vehicle. General Motors was sued for the defective design of the vehicle door which contributed to the severe brain injuries suffered by Jeffrey Jernigan.
The jury awarded $22 million in compensatory damages and $100 million in punitive damages. Immediately following the verdict, the court indicated that it would reduce punitive damages to $60 million in accordance with Alabama law which caps punitive damages at three times compensatory. Jernigan v. General Motors Corp. (Ala. Cir. Ct., Bullock County, 5/2/2002)
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