Accident Reconstruction

Deaths from vehicle accidents declined in the years of the late twentieth century, thanks to the widespread use of seat belts and a decrease in persons driving while intoxicated. Cars are now manufactured to higher safety standards, with design features such as airbags and crumple zones to help reduce the impact of a collision. Airplanes and trains are equipped with numerous safety features and have established emergency procedures that protect passengers in case of a mishap. When an accident does occur, it is important to investigate it thoroughly, especially if someone has been injured or killed or if a crime has been committed. Reconstructing an accident is a key element of such investigations.

The police clear vehicular accidents away as soon as possible to stop blocking the highway. Investigators, therefore, may not have the chance to visit the accident site. Instead, they use evidence to create a reconstruction of the accident from which they can try to work out the cause. Investigators require access to: photographs of the accident site; the police accident report, including eyewitness reports; and the wreckage of the vehicles. These sources provide information on velocities and positions of the vehicles; skid marks; impact damage to the vehicles; condition of the vehicle, especially the tires; the weather conditions; and the condition of the drivers.

The investigators first put together a general reconstruction scenario and then fill in the details from the facts available. Because the final positions of the vehicles are usually well established, they might work backwards from this point. If there is also hard information on the initial velocities and conditions, perhaps from a speed camera or a reliable eyewitness, a beginning and end point of the collision might be established. Then the task is to work out what happened between these two points.

The majority of vehicle accidents involve collisions—with another vehicle, a stationary object like a wall or tree, or a pedestrian. Therefore, the focus of the investigation is usually to analyze the collision itself from the reconstruction so the cause of the accident can be determined. There are two physical laws that guide this analysis, the law of conservation of energy and the law of conservation of momentum. Used together, these can often provide a detailed analysis of a collision.

The law of conservation of energy states that energy can neither be created nor destroyed, but it can be converted into different forms. A moving object, such as a vehicle, has kinetic energy. When it comes to a stop, it has zero kinetic energy. Under normal conditions, this energy is converted into heat through the friction between the brakes and the road. In a collision, the kinetic energy is dissipated in two main ways, via a skid, and in deforming the vehicles and their occupants on impact. A skid is an out-of-control type of braking and the analysis of any skid marks is a very important part of the accident reconstruction. Deformation of a metal object like a car involves work, the energy for which is the kinetic energy of the moving vehicle. Injuries to individuals are also brought about by transfer of kinetic energy.

Momentum is the product of the vehicle's mass and its velocity. A car traveling at 30 miles an hour has less momentum than one going at 40 miles an hour, and a truck generally has higher momentum than a car at the same velocity. When a collision occurs, the total momentum of the vehicles remains the same, but their individual momenta will change. If a car hits a stationary vehicle in a traffic queue (line), it will lose momentum, but the other vehicle will gain it. Overall, there is no alteration in momentum. The investigator can use the conservation of energy and momentum equations to try to establish facts such as initial velocity of the vehicles and so build a detailed picture of the collision.

The point of impact of the collision is an important factor and one that may be difficult to determine exactly. Tire marks, such as skids that suddenly change direction, however, can act as important clues. A head on collision might flatten the front tires, giving rise to a skid mark that suddenly terminates.

Examination of the vehicle wreckage, generally done off road, can also provide valuable information. For instance, under-inflated tires blowing out are an important cause of accidents. The blow point is often marked by a characteristic discoloration in the surrounding area. The hardness of the tire can also be measured using an instrument called a durometer. Its hardness profile shows if the tire has been run under-inflated. Metallurgic analysis of any fractures or dents in the car wreckage can give information on issues such as faulty welding or inferior repair work.

Accident reconstruction is particularly important in the case of large-scale accidents that involve many people, such as plane crashes, train wrecks, and even space shuttle disasters. The National Transportation Safety Board takes the lead in plane crashes and train collisions or derailings. Recovery of an airplane's black box, recording crew voices and instrument readings, is an important way to recover pertinent information about a plane crash. Conversations with air traffic controllers may provide information on what was happening on the plane at the time of impact. Witnesses may be asked to verify sounds or smoke coming from an airplane in trouble. These investigations can be quite complex, and can take a year or more to complete, especially in high-profile cases.

The National Aeronautics and Space Administration (NASA), and sometimes an independent review board, investigates space shuttle accidents, such as the explosions of the Challenger in 1986 and Columbia in 2003. Although space shuttles are not equipped with black boxes, the crew members are in constant communication with NASA crews on the ground, and information from the shuttles' computers is constantly fed to ground control and recorded. This information becomes vital to an accident investigation and reconstruction process, should it become necessary.

Accident reconstruction is a complex matter. Not only does information from many sources have to be integrated, gathering this information is sometimes difficult, even traumatic. And however well the reconstruction has been put together, it is still only a model, and will often be subject to challenge from experts acting for parties to the accident.

see also Airbag residues.