To reconstruct a vehicular accident, the engineer must determine the chain of events leading up to the accident and the vehicle dynamics before and after impact. Because physical evidence is time-sensitive, inspection of all involved vehicles is typically first on the agenda. Newer vehicles that have airbags are equipped with an event data recorder (EDR) in which critical information from the period immediately prior to airbag deployment is stored. Vehicle speed, engine speed, brake status, throttle position and air bag status can be captured via digital download from the vehicle after the accident. It is important to understand that event data will be overwritten if the vehicle goes back on the road without having been downloaded. The EDR is an example of high tech evidence gathering.
Some low tech information is available to the “trained eye” of the engineer that is just as critical. Headlight analysis looks at the condition of different filaments to determine if headlights (or taillights) were on or off at the moment of impact. The accompanying photo is of a halogen headlight bulb with two filaments. The low beam filament to the left is oxidized which indicates that it was burning at temperature in the presence of oxygen. This bulb probably developed a leak during the accident, allowing the halogen gas to escape. If the bulb continued to operate after the bulb developed a leak, the filament wire would have been consumed until it was broken. The high beam filament to the right is intact and shows no sign of discoloration. It was not on and has not been on since oxygen entered the bulb.
Similarly, seat belts are examined for stretch marks to determine if the vehicle occupant was belted at the time of the crash. Crush measurements, vehicle scratches (in the event of a rollover), tire inspection and skid marks are all used as inputs to the analysis of the speed and direction of each vehicle before and after impact.
Once the physical evidence has been examined, the engineer will review the police report and witness statements to re-create the chain of events. Finally, the engineer will apply engineering principles to the evidence gathered during inspection and validate the reports from the accident scene. For more examples of evidence gathering, inspection checklists, or questions on accident reconstruction, visit our website at www.cedtechnologies.com or click Case Manager to receive the curriculum vitae and rate sheet for our engineers.