Motor Vehicle Crash Investigation and Reconstruction

John C. Glennon, Jr., BSAT

[ Reprinted from Through the Gears Trucking Magazine, Volume VII, Issue 1, January 2001]

Accident investigation and accident reconstruction are commonly heard terms, but what exactly are they. The following discussion will attempt to shed some light on these processes.

After a motor vehicle accident occurs, particularly one resulting in severe injuries or death, it will usually be investigated and sometimes be reconstructed. There are two primary types of people that perform these duties, the police and independent investigators.

The police will investigate an accident with the intention of determining if any criminal action took place in the accident. Some of the things the police look for are speeding, hours-of-service violations, mechanical violations, alcohol use, drug use, etc. For example, if a truck driver is exceeding his hours of service and causes a serious accident because he fell asleep at the wheel, he will likely be criminally charged. If that accident results in a fatality, the charge will likely be homicide.

Independent investigators are normally hired by the insurance companies or attorneys of the parties involved in an accident. The job of the independent investigator is to analyze the causes of the accident so that liability and/or financial responsibility can be determined. These independent investigators are often off-duty police officers, retired police officers, automotive technologists, or engineers. However, investigators from many other backgrounds are also common.

An accident investigation can consist of many aspects. However, the investigation usually starts with an inspection of the accident site. At the site, measurements are taken of evidence left by the vehicles such as point of impact, final resting positions, skid marks, scrub marks, and gouge marks. These measurements are usually taken using electronic surveying equipment. With this equipment, a computer-generated scale diagram can be produced and may be used to reconstruct the accident.

Vehicle inspections are another typical part of the accident investigation. These inspections will usually include measuring the amount of damage and damage profile of the vehicles. This information will be useful if an accident reconstruction is performed. The mechanical components of the vehicles such as brakes, steering, tires, suspension, lights, etc. may also be inspected and tested to determine if the condition of these components was a causative factor in the accident. The investigation might also include looking at driver qualifications, auditing logbooks, or reviewing other motor carrier compliance related materials.

With the accident investigation complete, an accident reconstruction can be performed. Reconstruction is the process of using physics to determine the speeds of the vehicles, and/or their relative positions at different times during the accident sequence. Information such as pre- and post-impact direction of travel, length of pre-impact skid marks, post-impact distances moved, friction values for the various surfaces the vehicles traveled over, point of impact, impact angles, and weights of the vehicles are all used as inputs into the equations used to reconstruct an accident.

There are many types of accident scenarios such as head-on, rear-end, right-angle, or roll-over. Each of these different scenarios requires a different method to reconstruct. However, generally speaking, an accident reconstruction will determine how much of a vehicle.s speed is lost in each part of a collision sequence. Take, for example, a vehicle that skids off the road and strikes a tree. A reconstruction of this accident would use the vehicle.s damage profile to calculate the speed at which the vehicle struck the tree and combine that with the amount of speed lost while the vehicle was skidding to determine the pre-braking speed of the vehicle.

A slightly more complex accident reconstruction involves two vehicles first skidding, then colliding with each other, then sliding to a stop. For this type of accident the method used is called the Conservation of Linear Momentum, which takes into account the weights of the vehicles, the angles at which they collided, and the places where they came to rest. To be more specific, take the example of two-vehicle collision where one vehicle is traveling due south the other vehicle is traveling due west. After these two vehicles collide, the rules of physics tell us they will move generally southwest, with all of the southward momentum resulting from the southbound vehicle and all of the westward momentum resulting from the west bound vehicle. If the reconstructionist knows how much each vehicle weighs, how far each vehicle moved south, and how far each vehicle moved west, then he can calculate a collision speed for each of the vehicles. This collision speed for each vehicle can then be combined with its speed loss from pre-collision skidding to calculate its pre-braking speed.

Continuing with this right-angle accident reconstruction, the pre-braking speeds can be used to both look at time-distance relationships before collision and to pose .what-if. questions. For example, if one of the vehicles was found to be speeding before collision, the reconstructionist could pose the question what would have happen had that vehicle not been speeding. By moving that vehicle at its calculated speed back from the initial braking point by say 1.5 seconds for a normal driver perception-reaction time, the reconstructionist can determine a point of perception. Then by asking what if that driver had been traveling at the speed limit, had taken 1.5 seconds to perceive and react, and had locked the brakes, the reconstructionist can determine if the accident could have been avoided if the driver had simply obeyed the speed limit.

The discussion above gives just a brief overview of the processes of accident investigation and accident reconstruction. For a more complete discussion, the reader is referred to, where several good accident investigation and reconstruction publications are offered.

About the Author

John C. Glennon, Jr., is a forensic automotive technologist who performs crash reconstruction and detailed vehicle testing for trucking companies, insurance companies and lawyers involved in investigating and litigating motor vehicle collisions. He has a B.S.A.T. degree in Automotive Technology, he is a triple-certified Master Automotive Technician, and he holds a Class A CDL in the state of Kansas.


NOTICE: This copyrighted internet paper and our others, which can be accessed by clicking on OTHER PAPERS, are considered dynamic entities that may be updated or expanded at any time. If you have a critique, comments, additional information, or would like to see additional material in the paper, please email us at

john c. glennon, truck crash reconstruction expert, automotive mechanical failure analysis expert, motor carrier safety and compliance expert, truck driving standards expert, truck maintenance analysis expert, runaway truck crash expert, brake failure analysis expert, air brake failure analysis expert, wheel failure analysis expert, hub failure analysis expert, tire failure analysis expert, truck underride crash expert, auto event data recorder analysis expert, truck event data recorder expert, diminished value inspections, lemon case expert
Crash : John C. Glennon, Chartered Contact Us Links Home