Automobile accidents are still a major source of injury to pedestrians, drivers, and passengers, although mortality has gone down owing to the introduction of seat belts and other safety measures such as air bags. The forensic investigator is usually called in when a crime, such as a hit and run incident, speeding, drunk driving, or even homicide, is suspected. An investigation is also necessary when insurance claims for damages by the injured parties arise from the accident. A medical examination and assessment of the injuries sustained in an automobile accident form a vital part of the total investigation of an automobile accident. The medical and pathology evidence can be integrated into physical evidence , such as tire marks or impact damage, at the scene of the accident to help reconstruct what happened before, during, and after the accident.
An automobile may collide with another vehicle or with a stationary object like a wall, lamp-post, or a pedestrian. The amount of damage inflicted depends on many factors such as the weight, velocity, and condition of the vehicle; the state of the road; and the condition of the driver. Injuries occur when the momentum and kinetic energy of the vehicle are brought to zero on impact, since the energy is transferred to the skeleton and surrounding tissues of the victim(s). There are three types of victims in an automobile accident: pedestrians; cyclists (motor or pedal); and the occupants of the vehicle, be it the driver or front or back seat passenger(s). Most often, it is the pedestrian who is most vulnerable to injury in an automobile accident, particularly in areas where potential contact between pedestrians and vehicles is high, such as at crossings or in built-up areas. Pathologists note characteristic patterns of injuries in these different victim groups that can be very revealing about the circumstances of the accident.
Pedestrians generally suffer two kinds of injuries when they come into contact with moving vehicles. Primary injuries arise from contact with the vehicle itself. Secondary injuries occur when the impact with the car forces the pedestrian into contact with some other object or surface, such as the road itself. Often it is the secondary injuries that do the most damage and which are the cause of most fatalities arising from automobile accidents.
For an adult hit by a car, the bumper generally strikes them first, around knee level at the front, side, or back of the leg, depending on their location relative to the vehicle. Further contact, generally with the hood of the car, produces injuries further up the body, usually to the pelvis, thigh, or hip. If victims are hit by a larger vehicle, such as a truck, the corresponding primary injuries will be higher up the body. If a child is hit, then the primary injury pattern is also higher up the body. Older people and children are more vulnerable to being hit by a car because the former tend to be less mobile and the latter may not be sufficiently vigilant or able to judge the speed of an oncoming vehicle. What happens after the initial impact depends on how fast the vehicle is going. Up to 10 mph, the pedestrian will likely be thrown off the hood of the car onto the road. If the car is traveling up to 35 mph—just beyond the speed limit for most built-up areas—then the victim tends to pitch forwards on the hood and the victim's head may strike the windshield. At higher speeds, the victim may be flung into the air, over the car, and hit the road with some considerable force. A pedestrian who may survive a 30 mph impact with a car could well be killed if that same vehicle is traveling at 40 mph. The pathologist's evidence can be vital in accidents caused by speeding.
Automobile accidents produce a wide range of secondary injuries to pedestrians. Skidding across the road will produce extensive abrasions. Direct contact often causes fractures of the skull , spine, and limbs. It is usually the internal damage associated with such fractures that is lethal rather than the fracture itself. Brain damage, even without a skull fracture, is common if the moving head hits a road surface.
Being "run over" by a car is actually quite unusual. When it does occur, though, injuries can be very extensive. Their nature depends on what part of the body is involved, the speed of the vehicle, and its weight. The skull may be crushed, and internal organs disrupted. If the car runs over the victim's chest, there will be multiple fractures to the ribs. Sometimes the rotation of the wheel will strip off a huge area of the skin, giving rise to a characteristic "flaying" injury. Occasionally, someone is run over deliberately, but more often it happens when someone is knocked down accidentally and then run over if the car cannot stop in time.
Most automobile accidents involve the front or front sides of the vehicle. Those occurring by impact from behind or from the side, which may cause the car to roll over, are somewhat less common. The front seat occupants, if unrestrained by a seat belt, will continue moving as the car decelerates on impact. The head and face hit the windshield, possibly causing skull fractures and brain damage. The driver's chest and abdomen can hit the steering wheel, causing damage to the ribs, heart, and liver. The force of impact may even tear the body's main artery, the aorta, an injury that is generally fatal. The impact of the crash on the legs is transmitted upwards, potentially causing multiple fractures of the legs and pelvis. On very severe impacts, the front occupants may even be flung through the windshield to hit the road and sustain secondary injuries. On a fast-moving highway, such victims may be in danger of being run over by other vehicles. The back seat occupants are prone to similar, but usually less severe, injuries through impact with the front seats and the front seat occupants. If the impact is very severe, they may be flung through the windshield.
Seat belts, worn by both front and back seat occupants, have undoubtedly reduced fatalities from road traffic accidents. They work by preventing contact between the occupant's body and forward obstructions like the steering wheel and windscreen. They also spread the force of impact on the body and increase deceleration time slightly so that the force per unit area of body is less, which reduces the severity of any injuries. A seat belt also stops an occupant from being flung out of any doors or windows that burst open on impact. Airbags, a more recent development, provide a "cushion" between the vehicle and any object with which it collides. This also protects front seat occupants from injury. However, some critics have argued that the presence of an airbag produces a sense of complacency among drivers, making them less careful than they should be because they assume the air bag will protect them in the event of an accident.
Motorcyclists generally fall from their machine onto the road when in collision with another vehicle or object. This causes, at the very least, severe abrasions. Injuries to the limbs, chest, and spine are also common when the motorcyclist becomes entangled with the machine, which may fall on top of the motorcyclist, or with another object. Head injuries are a common cause of injury and death among motorcyclists involved in an accident, even if a helmet is worn. In many countries, a helmet is compulsory and its presence or absence may be a factor in any criminal case or litigation. But, protective clothing and a good helmet can go a long way to reducing the severity of injury to motorcyclists in an accident.
Most bicycles travel at relatively low speeds, which means that most injuries sustained in an accident are mild to moderate. However, collision with a vehicle traveling at low speed may well produce fatal injuries if the cyclist is thrown against the vehicle and then falls onto the road. Cycles are inherently unstable and even the most gentle of collisions is likely to make the rider fall off, potentially into the path of a moving vehicle, when they may be "run over" and sustain severe, and potentially fatal, internal injuries.
Whatever the type of victim, be it pedestrian, car occupant, or cyclist, the principles of the forensic medical exam are the same. The investigator will first, of course, offer medical assistance to living victims. Then an external exam is done to note all the injuries, recording, measuring them with a ruler and taking photographs. Sometimes tire marks are present in the form of bruising or abrasions and these can be helpful in identifying the vehicle used by a hit and run driver. The victim's clothing should also be examined for trace evidence , such as glass , paint, or rust, which might help identify the vehicle involved. It is always possible that alcohol or drugs have contributed to the accident and blood samples are often taken from the victim and drivers, if they are present, for laboratory analysis.
see also Accident reconstruction.