LOW IMPACT COLLISIONS
Injuries received from low impact collisions cost insurance companies millions of dollars every year. General Motors Corp. has reported more than one third of motor vehicle injuries occurred below 20 kph. Of these incidents, numerous are believed to be fraudulent. Some insurers have recently initiated programs in an effort to detect and reduce the number of fraudulent claims. Methods include surveillance, investigation into personal histories and the use of engineers to assist in determining the velocity change experienced during the collision by the party claiming injury. The resulting velocity change can then be compared to data collected through a growing database of volunteer subjects.
There are two main bumper systems available on vehicles today, the "isolated" version and the "plastic" version. The "isolated" bumper systems employ rigid bumpers mounted to the vehicle frames via isolators. The "isolators" are small shock absorbers with a piston inside a cylinder. The outer portion of the cylinder contains high pressure nitrogen gas and the inner portion contains oil. A floating piston is positioned inside which separates the gas from the oil and acts as a movable barrier. The oil is allowed to flow through a small hole and a metering pin as the bumper compresses the piston. As a result of this design, the bumper moves out more slowly after compression thus rebounding with less violence than a spring.
The "plastic" version of bumper systems is typically made of high density foamed polystyrene or polyurethane mounted on a rigid plate to the vehicle frame. An open cell, honeycomb plastic structure is also used. Due to the natural elastoviscous response of foamed polystyrene and polyurethane these bumper systems also rebound with less violence than a spring.
Extensive testing and study has been conducted on the "isolated" bumper system. As a result, velocity change involved in low impact collisions may be determined when one of the two vehicles is equipped with isolator shocks. It becomes more difficult to determine velocity change from damage or lack of damage on rigidly mounted steel, foam and honeycomb core bumper systems although some recent studies available allow fir determination of velocity change in some circumstances particularly those involving override and underride. Future studies will hopefully improve engineering abilities in this area.
Information reported by the witnesses to the accident, coupled with the physical characteristics of the site of the accident, can also assist in determining maximum velocity change which could have occurred as a result of an accident. This is particularly useful in impacts which occur after vehicles have come to a complete stop and subsequently accelerate. These situations typically occur at intersections where vehicles come to a complete stop following other vehicles. The slope of a ramp or a roadway, idle speed, forward and even rearward acceleration rates have also been useful in determining maximum speed change experienced during an impact.
Numerous studies have been performed subjecting human volunteers to rear end collisions of varying velocity change. The subjects involved were both males and females of varying age groups including some with a predisposition and/or history of neck and back injury. The current database includes over 300 volunteers. No lasting injuries have been reported at velocity changes below 8 kph. However, a person with prior neck problems could experience some degree of injury at impacts below this threshold due to limited natural neck extension and lower damage level due to scar tissue.
Further analysis has shown that injury can be expected for a normal, healthy, average person at speeds in the range of 12 to 27 kph depending on natural extension, individual strength and initial muscle tensing. Position of the headrest can further reduce the potential for injury by preventing hyperextension and reducing strain on the neck muscles. Speeds above 60 kph have been successfully survived under ideal conditions.
Other factors which can influence the probability of injury include; the principal direction of the force applied to the vehicle, seatback position and design, occupant neck length, head and torso position, knowledge of the impending impact, previous injury history and other occupant physical characteristics. Clearly each individual case must be analyzed in detail.
Overall, a normal person should be able to withstand a velocity change of 8-11 kph and up to 16 kph with a properly positioned headrest and/or knowledge of the impending impact. Persons with mild preexisting neck problems should be able tolerate 8 kph speed changes provided the headrest is positioned properly. For a detailed analysis of an accident of this type, please call the Walters office nearest you.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2