We are seeing more motor vehicle accidents where the local authorities conduct an accident reconstruction. These investigations are probably occurring more frequently because of the black box now found in many vehicles. The black box provides pre-collision information on speed, throttle position and brake status.

Often though, the black box information is insufficient and the accident reconstructionist must combine that data with traditional techniques based on the laws of motion. One method uses the black box data and the Law of Conservation of Momentum.

In this context, the principal of the Law of Conservation of Momentum can be stated as follows:

  • When vehicle one strikes vehicle two, all other things being equal, the faster vehicle one is going the more it will knock vehicle two off-track.

One type of crash where this technique is used almost always involves death or serious injury. A vehicle (let’s say an SUV) makes a left turn in front of an oncoming motorcycle, resulting in the motorcycle striking the SUV. The SUV has a black box from which its speed prior to impact can be determined. The motorcycle, though, has no black box. The driver of the SUV says that when he made his turn, the motorcycle was far away, and therefore must have been speeding. This motorcyclist has no memory of the accident. This type of collision often occurs suddenly and there are no skid marks or other pre-impact marks on the road. So, was the motorcycle speeding?

The difficulty in determining the motorcycle speed using the Law of Conservation of Momentum is that it involves a lot of variables, some of which are unknown, and must be estimated. The variables include:

  • The velocity of the SUV just prior to impact. (Velocity is a vector quantity consisting of speed and direction).
  • The velocity of the motorcycle just before impact.
  • The weight of the motorcycle and the SUV.
  • The exact orientation of the motorcycle and the SUV just prior to and immediately after impact. These are known as the vehicle approach and departure angles.

The weight of the motorcycle (and rider) as well as the weight of the SUV is available. The black box provides the speed of the SUV prior to impact. Post collision markings on the road permit the investigator to estimate the departure angles of the vehicles. But what about the vehicle approach angles, particularly that of the SUV? How was the SUV oriented just before the collision?

Because of the absence of pre-accident markings on the road, the investigator may not be able to precisely determine the approach angle of the SUV, making it difficult to determine to what extent the motorcycle knocked the SUV off-track.

The reconstructionist will often estimate the approach angle, but this is often nothing more than a guess. Unfortunately, even a slight error in the approach angle, say, 20 degrees or so, can result in a motorcycle speed which is substantially higher than it actually was.

The lawyers at Liever, Hyman & Potter, P.C., are concerned for the safety of automobile drivers and passengers in Berks County and Schuylkill County and serve accident victims and their families in Reading, PA, Pottsville, PA, and throughout Eastern and Central, Pennsylvania.

From the desk of Edward E. Houseman, Esquire.

Latest Posts