This is a common task often initially performed by the investigating police officer, or afterward by the accident reconstructionist. Unfortunately, some “experts” have fundamental misunderstandings about skid mark analysis that result in huge calculation errors, which often go unnoticed. If you have hired a reconstructionist, ensure that they have:
Identified the marks as skid marks. Not all tire marks are caused from braking. Yaw marks for example, are created when the vehicle is traveling at an angle different than the direction it is facing. (this is called the slip angle)
Skid Marks
Yaw Marks
Yaw marks are differentiated from skid marks by the alignment of dark bands within the mark. They are aligned at an angle to the direction of travel for yaw marks, and parallel to the direction of travel for skid marks.
Skid Marks have Parallel Dark Bands
Yaw Marks have Dark Bands Aligned at an Angle (click photo for larger resolution version)
Measured or assumed a friction factor, and explained why. Different surfaces call for different friction factors, and these friction factors may vary even on a single surface. For example, passenger car tires on travelled asphalt range from 0.6 to 0.8. (“Friction Applications in Accident Reconstruction”, Warner et al.) Alternatively, if the surface is suspected to be different than reported in research, or relevant research can’t be found, acceleration tests can be performed with a similar vehicle at the site.
Measured the skidmarks correctly. This is the most common error made during the reconstruction. Skid marks may be of equal or different lengths, or one or more tires may not produce skid marks. The reconstructionist must explain the lack of any skid marks. For instance, a tire was not braked (many inexperienced motorcyclists fail to use their front brakes in an emergency situation), or there was a brake failure (which can be revealed from a vehicle inspection). This article only addresses the situation where all tires leave skid marks (of equal or unequal length).
When all (4 for passenger vehicles) skid marks are of equal length, then that length is used. If the skid marks are of unequal length, then there are two methods to calculate the possible speed of the vehicle.
use the average length of the 4 skid marks. This method produces a conservatively low estimate of the vehicle’s traveling speed.
use the length of the longest skidmark. This method produces a conservatively high estimate of the vehicle’s traveling speed.
Pitfalls to avoid
The following examples come from real cases we have worked on, and represent an extreme lack of understanding regarding the fundamentals of skid mark analysis. If your reconstructionist does any of the following, contact us.
Adding skid mark lengths together.
Confusing yaw marks and skid marks. See the photos above for examples.
Using friction factors to calculate a vehicle’s acceleration. A vehicle’s acceleration (increase in speed) is not generally equal to its deceleration (decrease in speed, or braking).
How this can help you as a litigator
If you spot any of the above errors, or just think that the speed estimate given by the other side doesn’t “feel right”, contact a Crashteams reconstructionist. They will be able to review the other expert’s work, and provide you with a verbal or written report detailing exactly what that expert did wrong, along with the correct method and correct results.
Speed from skid mark calculations
Simple skid mark calculations can be automatically performed at the following link. Be careful to avoid making any of the mistakes discussed above!
Crashteams collision reconstruction experts are often called upon to assess the forces involved in collisions where there is little visible damage to the involved vehicles. The reason we are assigned to these cases is simply that there is a suspicion that there was insufficient force in the collision to generate the injury causing mechanism. Normally the orientation of the collision is a same direction rear ender. Normally, the party advancing a claim for injury is in the struck vehicle. The technical process of determining the magnitude of the collision involves inspection of the involved vehicles. We inspect the bumper surfaces for signs of permanent deformation. We inspect, if the vehicles are so equipped, the bumper isolators for signs of sudden compression.
An SAE paper entitled “Analysis of Human Test Subjects Kinematic Responses to Low Velocity Rear Impacts” by McConnell indicates that even before the neck is hyperextended, the injury mechanism begins to occur. This means before the head accelerates any great distance rearwards to the point where the rearward excursion is limited by the positioning of the head rest, the injury has already begun. However, the injury mechanism does not stop there.
In addition to the legal requirements, any expert worth his salt should have the “show the work“ documents. In other words, there should be electronic or hand calculations to show how the answer was achieved. Keep in mind, most experts are either taught to use a series of equations, already derived, for application on different cases ( police trained ), or trained to derive the applicable equations from first principles ( engineer reconstructionists). Regardless of background training they should explain the equation they used, why it was used, and where it came from. If an expert accident reconstructionist cannot do that, he/she is obviously not an expert.