Multiple collisions, in which a vehicle is submitted to one or more successive collisions after the initial collision, are of special significance as far as the types of occupant injuries and the injury severity are concerned. One of the primary concerns for multiple impacts is the risk of the first impact resulting in the occupant being out-of-position (OOP) for the second crash. In addition, having injuries from a second impact superimposed upon injuries from the first impact may significantly alter the nature and extent of injuries, depending upon the circumstances. In some instances, the doctor will be confronted with a patient who has been involved in a multi-vehicle collision and has been requested to determine if the first or second crash was responsible for the injuries or the respective percentage of causation. The doctor’s opinion may be needed to determine apportionment for each collision. If two insurance carriers are involved, they may want to pass the blame for injuries to the other carrier. From a biomechanical perspective, multiple collisions are very challenging. Several issues that need to be considered:
A) delta-V for first impact and the subsequent crash(s)
B) mass ratios of the vehicles involved for each impact
C) vehicle rotation for each crash, if occurring, and if so, how severe
D) types of vehicles involved (i.e., SUV vs. a passenger car in a side impact). Height issues between the two vehicles and aggressive designs can result in differing injury patterns.
E) intrusion extent into the occupant compartment for each vehicle, if any.
F) contact point on the vehicle relative to the seating position of the occupant and proximity to the contact area, and whether the point of impact (POI) was rigid or soft. Rigid pillars and smaller structures will cause much higher acceleration levels than softer impacts or by larger objects capable of distributing loading over a larger area of the body.
G) times of occupant acceleration. Longer impacts are better for occupants
H) OOP issues for occupant at the time of each impact. Both impacts may have OOP issues.
I) Second injury superimposed on first injury. Doctor needs to consider each impact with its own characteristics. For example, did each impact involve a head strike or did one impact cause inertial loading only? Need to consider the surface size and energy absorbing quality for each impact as well.
J) Type and severity of first injury can make second impact more or less likely to cause other injuries.
K) Patterns of injury related to vector of each crash.
L) other human and vehicle factors.
Temming and Zobel used a Volkswagen database of 1,620 occupants who suffered cervical spine injuries, concluding that about 23.5% of occupants in passenger cars were involved in at least one additional collision and 5.6% of occupants had a third impact following the initial crash. Another study found that multi-vehicle collisions occur in about 15% of total crashed, and the second collision speed has been found to be just as high as or higher than the first crash 43.2% of the time. An example is the rear-end collision in which one car is pushed into a vehicle ahead, and is struck on the side of the vehicle in the intersection, starting a chain reaction of traumatic events. Otte et al. concluded that injuries may be caused by almost all parts of the interior, due to actual impact situation and the consequent relative motion of the occupants.
Fat et al. analyzed CCIS-UK data of 1,295 crashes looking for multiple impacts. Their study found that a secondary impact with another vehicle occurred in 22.6% of crashes and an additional 6.1% were involved in a third impact. The study concluded that there was a higher risk of occupants being seriously injured in multiple impacts, with the head having the highest frequency. The most common double impact events happened in frontal crashed followed by side impacts or side impacts followed by another side impact.