Putting the Pieces Together Vehicle Accident Reconstruction

Daniel N. Olson, M.S.M.E., P.E.

Imagine if all of our motor vehicles had black boxes inside of them, just like airliners do? What if the box kept track of how fast the vehicle was going, when the brakes were activated, where the throttle position was, where the steering wheel was, what the weather was like, and the road conditions. What if there was a video camera aboard every vehicle recording the view of the road ahead? What if intersections had video cameras to record traffic and collisions as they occurred? We could then easily figure out who entered the intersection first, who ran the red light or who was speeding and accurately assign the responsibility for the damage and injuries. Aw, but such is not the case and I don't think many of us would submit to this type of surveillance.

Short of high technology, when there is a collision, what type of information do we need to reconstruct the accident?

Witnesses Interviews
There may be witnesses. Hopefully there were survivors from the accident who can tell us a lot about what went on. However, be aware they have been through a traumatic experience and may not accurately recall what happened. Bystanders can tell you what they observed but their interpretation of the events, especially the speed of a car, is not reliable except in relative terms.

Vehicle Damage
The vehicles hold a lot of good clues. The crash damage can tell us what part of one vehicle hit the other. Scrape marks reveal how a vehicle rolled. How parts failed can tell us about the geometry of the collision. Computer programs are available that estimate the energy absorbed by a crashed vehicle. From the energy absorbed, we can calculate speed lost in impact. Often, we can make an analysis of the pre-crash road worthiness of the vehicle.

Clues at The Scene
Skid and tire marks on the road can tell us when a driver reacted, the direction of travel and sometimes can indicate speed. Gouge marks and any kind of physical evidence (i.e., a path of fluids that might have drained out of the vehicle) can tell us where the collision took place and the path of the vehicles. Damaged foliage, damage on signs, and marks on curbs can help us determine the path of the vehicle.

Resting Location of Vehicles
Some of the most important information that often gets overlooked is where did the vehicle(s) come to rest. All too often vehicles are picked up and the scene is cleared before anyone has properly documented that scene. Many of our uniform traffic collision reports completed by law enforcement officers often only show a generalized location of where the vehicles came to rest. This general data is usually not precise enough to determine the path of the vehicles from the collision point to where they came to rest. The path of the vehicles after the crash is just as important as the path of the vehicles into the crash, when reconstructing the accident. This data can be used to determine speed and direction of the vehicles as they entered the collision.

Measuring & Documenting the Scene
With the resources of a camera, tape measure, a measuring wheel, and a heading compass, the measurements needed to establish the resting location of the vehicles should be fairly easy to gather and document. Pick two reference points around the scene that are permanent. You can choose from fire hydrants, manholes, curb and gutters, street signs, large trees, electric poles, corners of buildings, guardrails, bridges, sidewalks, and intersections. Then pick out one point on each vehicle. Measure the distance from that point to both of the reference points that you chose. Draw a sketch that shows your two reference points and the lines between the reference points and the vehicle points. Now choose another point on each vehicle and measure the distance from the second point to at least one of the reference points. Try to pick vehicle and reference point combinations that will define lines that are angled from 30-150 degrees from each other. That's it! With that sketch, a reconstructionist can accurately plot where the vehicles ended up.

Determining resting location is just one important piece of the puzzle needed to put the accident or the sequence of events back together. When that piece is missing or uncertain it is more difficult and sometimes impossible to reconstruct the collision.