Accident Reconstruction
What Can It Determine ?

The field of accident reconstruction is rapidly growing in both criminal and civil traffic crash cases. There are numerous reconstruction techniques that may be utilized to prove or disprove certain facts of a given case.

FORCON accident reconstruction consultant Steve Chewning has prepared the following article to familiarize Bridge-The-Gap recipients with the most frequent uses of accident reconstruction analysis.

Speed Analysis

With the proper approach, the reconstructionist can determine speed from skids, scuffs, flips, vaults and falls through mathematical formulas which have been derived from the applicable physics principals. The sooner after the crash that speed analysis is undertaken, the higher the likelihood of success.

The variables to be considered and accounted for in speed calculations from a set of skid marks are:

a. Distance of skid in feet.

b. Number and identification of tires skidding (front or rear)

c. Coefficient of friction on the surface to include the grade of the roadway. Depending on where the reconstructionist was schooled, the term used may be drag factor, friction coefficient, surface resistance factor, or surface drag, but all of these terms are synonymous.

A scuff mark, also called a critical speed scuff, is produced by a car that is out of control and actually moving forward and sideways in rotation prior to a crash, The centrifugal force pulling the car outward from the attempted turning arc has become greater than the centripetal force holding the car in the intended turning arc. The car, therefore, moves outward into a wider radius until the forces are once again balanced and the car can complete the turning motion. By calculation of the arc, along with the needed variables, the speed at which the forces balanced can be easily determined.

The impact speed of colliding vehicles can be determined by conducting an analysis of momentum involved in the crash and redirecting the total momentum to the original component at impact. The variables involved in undertaking momentum analysis are as follows:

a. The weight of each vehicle including occupants and cargo.

b. The speed of each vehicle after impact (determined by using the basic speed calculation method on the movement after impact to final rest).

c. The angle of approach to impact of both vehicles. This angle is determined from pre-impact skid paths or damage thrust angles to the vehicles.

d. The angle of departure from impact for both vehicles.

Impact Location and Angle Analysis

Examination of the roadway evidence such as gouge marks and tire marks can be matched to the specific part of the vehicle that produced the mark. The parts matched in conjunction with the damage pattern on both cars can show the location of the impact within the roadway and the angle of each vehicle on impact.

Time and Distance Studies

There are numerous applications of time and distance studies which may prove beneficial. The sight distance of a particular scene is often an issue. A time and distance study can provide information of how many seconds the driver might have had to see and react to a particular incident from the limit of the sight distance to the point of no escape or to the point of impact at various speeds.

As an example of the technique, the distance from a hill crest to impact point can be determined and calculated along with the posted speed limit (or any speed chosen for use in the calculations depending on the circumstances) to determine if a vehicle cresting a hill could have stopped prior to impact. If the calculations prove that the vehicle was past the point of no escape when it reached the point of possible perception of the hazard, the complexion of the case may totally change.

Lamp Condition

Some of the most frequent allegations in certain types of crashes are that the other vehicle had no lights, that it failed to dim the lights, or that the brake lights or signal lights failed to function.



In cases where there is impact shock in the area of the questioned lamp, there can usually be a determination of "on" or "off" relating to a specific lamp.

There are numerous indicators that allow determination of the lamps' condition at the time of impact. All of the indicators are generally based on the fact that a tungsten filament becomes ductile when illuminated and will distort when subjected to acceleration or deceleration associated with impact, and that an incandescent filament has a chemical reaction when exposed to oxygen.

It is also possible to determine if a lamp was burned out prior to the crash. This finding may support a claim of no signal lamp, no brake lights, etc.

Speedometer/Tachometer Analysis.

In some cases the needle of a speedometer or tachometer may be stuck at a particular reading after a crash. It is important either to validate or disprove the reading of the speedometer/tachometer in such cases.

In head-on cases an examination of a speedometer or tachometer under magnification and high intensity ultraviolet light may determine if fluorescent tracings of the needle indication at impact are present. This examination technique may be very powerful evidence either to prove speed or to disprove the allegations made by other parties.

Driver Identification

The identification of the driver of a vehicle can be a question in many crashes where the occupants are ejected from a vehicle and later cannot or will not provide information as to the identity of the driver.Matching of the injuries on the individuals in the vehicle with the damage to the passenger compartment and the crash facts can create a strong case as to the identity of the driver or the passenger.

Closing Thoughts

As you can see from the information presented in this article there is a great deal of useful information that can be determined by an experienced accident reconstructionist provided the necessary data is available. Much of this data is only available at the accident scene immediately following the accident. A thorough police report, good accident scene photos, and detailed photographs of the vehicle damage can be, and often are used to perform some of the analyses discussed in this article. However, having a qualified accident reconstructionist investigate the accident scene and gather the field data as soon as possible after the accident can often insure having an important piece of data that might have otherwise disappeared.

The key to making a determination with respect to lamps, as in many other aspects of reconstruction is preservation and/or documentation of evidence before the vehicles involved are destroyed. The need for the examination should be considered sooner rather than later in the claims process.

Stephen B. Chewning

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