Rollover Crash Study - Vehicle design and occupant injuries

Monash University Accident Research Centre - Report #65 - 1994

Authors: G. Rechnitzer & J. Lane

Full report in .pdf format [72.1MB(warning large file)]

Abstract:

About 19% of the fatal crashes in Australia involve a rollover, with this crash type identified as the major contributor to serious spinal injuries resulting in paralysis.

The project aim was to investigate the correlation (if any) between vehicle design and the nature and severity of occupant injuries. The project involved a major literature review; and the detailed investigation of 43 rollover crashes with injury severity ranging from non injury to fatal.

The main conclusions based on the literature review and re-analysis of results from experimental roll-tests, was (contrary to reported conclusions) that the weight of evidence is in agreement with a relationship between roof crush and occupant injury.

The crash investigations generally supported these conclusions, but also identified roof design factors which could lead to spinal injuries in cases of low levels of intrusion. Also highlighted was the problem of partial ejection of seat belted occupants resulting in the risk of crushing head injuries; and that vertical roof crush in itself was not necessarily an indicator of rollover severity (as has often been asserted).

Recommendations for vehicle design changes are made, including: side window integrity; improved roof framing strength; modified geometric design of door/roof framing; interior energy absorbing padding; improved restraint system design and improved door integrity. Additional experimental and analysis work is recommended to clarify occupant kinematics in rollovers and resultant head and spinal loads.

Summary

About 19% of the fatal crashes in Australia involve a rollover. In addition, these types of crashes have been identified as the major contributor to serious spinal injuries resulting in varying degrees of long term paralysis and disability.

The aim of the project was to investigate the correlation between vehicle design and the nature and severity of occupant injuries in rollover crashes. Ale project involved a major literature review, including a re-appraisal of findings from other researchers; and the detailed investigation of 43 crashes ranging in severity from non injury to fatal, including cases of spinal injury involving paralysis.

The main conclusions from the literature review are:

Rollovers are recognised as serious accidents, with estimates of incidence variable, depending on crash classification and severity of injury, but are in the range 3% to 12% of serious injury accidents in general. Rollovers are largely single-vehicle accidents and occur predominantly at high speed, on rural roads and at night. Severe injuries are particularly associated with ejection, although this may be less significant in Australia, because of the high belt-wearing rate.
In restrained occupants, injuries occur, in descending order, to head, upper limbs, chest, lower limbs and neck, but, for AIS3+, head and chest predominate
Rollover fatality rates vary significantly for different vehicle types (and makes), from a low value for a 'large cars' to over six times this rate for 'utilities". This reflects partially the propensity to rollover, a function of 'Vehicle Stability Factor'.
At the start of most rollovers, the vehicle has a significant lateral velocity. The occupants become displaced from their seats and move towards and may impact the roof and may impose high loads on glazing and doors.
'Re main factors related to injury severity- the rollover event, ejection and vehicle speed - are interrelated, but each makes an independent contribution. Analysis is impeded by the lack of a measure of accident severity applicable to rollover.
From experimental rollovers (though open to criticism), dummy neck loads were significantly lower in cars with strengthened roofs, than in cars with standard roofs.
Experimental rollovers used standard dummies developed for frontal impacts, which may be inappropriate for rollovers, suggesting the need for a 'rollover dummy'.
In mass data and other crash collections, the weight of evidence is in agreement with a relationship between roof crush and occupant injury. There is a convincing relationship between rollover and spinal cord injury. Finally, there is strong evidence of a connection between local roof crush and spinal cord injury.
Experience from road and track racing indicates that rollcages have been highly effective in injury prevention in severe rollovers.

The main conclusions from the crash investigations are:

Ejection is a significant factor in fatal cases. Of the 13 fatalities, ejection occurred in approximately 50% of the cases (seat belts not worn), with the vehicle rolling on the ejected occupant in over half of these cases. Lack of roof integrity on certain vehicle models (particularly 4"s) can result in partial or full ejection of the occupants.
Current seatbelt designs are only partially effective in rollover crashes, providing little restraint against partial ejection and head excursion outside the vehicle and the risk of head injuries from contact with the road or being crushed between the vehicle structure and road surface.
Some seatbelt buckle designs may be deficient and unlatch during the rollover.
Nearly all roof structures and framing are unpadded and contribute to occupant head injuries, including scalp lacerations, skull fractures and brain injury.
Severe spinal injuries can arise from two main vehicle design related factors: The first is simply mechanistic, arising from vertical and lateral roof intrusion. The vertical space left for the occupant can be significantly less the normal sitting height, thereby resulting in high bending and compression loads on the spine. The second mechanism relates to impact loading of the head with the ledge formed by the underside of the roof and door frame, with consequent head and spinal injuries.
Rollover severity is reflected by roof crush combined with general levels of vehicle structure deformation. Roof crush alone may simply reflect roof strength and design.
Severe injuries (to non-ejected occupants) only appeared to occur to occupants seated on that side of the vehicle where significant roof contact with the ground/road occurred, or where there was significant roof crush. Roof crush is only really relevant when it occurs near the particular occupant's seating position.
The actual injuries received by occupants in a rollover, are also partially dependent on "luck" - e.g. the exact position of the occupant's body as the vehicle rolls.

Recommendations

The following vehicle design changes are recommended to reduce both the risk and severity of serious injuries arising in reliever crashes.

(i) Maintain side window integrity (by plastic glazing) to prevent head excursions outside the vehicle.

(ii) Increase roof framing and A and B pillar strength, for axial loading and side-sway loading, and require a minimum standard for roof integrity.

(iii) Provide interior energy absorbing padding to head contact surfaces - the roof itself and the framing above the door.

(iv) Modify the design of door/roof training to reduce risk of occupant's head being able to 'lock in' against this framing and hence result in excessive spinal loading.

(v) Improve the performance of seat belts to reduce vertical movements of occupants in relievers.

(vi) Improve door integrity and add energy absorbing side padding.

Additional Research

Additional experimental and analysis work is required to clarifying occupant kinematics in relievers and resultant head and spinal loads. Projects include vehicle drop tests, computer simulation and modelling, and investigation of restraint system performance.

Sponsor: VicRoads