Speed Determinations in Collision Cases

Speed determinations in a motor vehicle collision are frequently critical. Skid marks are a common way of attempting to determine speed.
There are formulas that can be used to determine the minimum speed of a vehicle before a driver applied the brakes. It is not uncommon, however, that vehicles traveling at a significant speed only leave a very short skid mark simply because the brakes were not applied until the last moment. However, where skid marks are laid down over a long distance then the so-called straight-skid speed formula may give some good evidence of actual speed.

In addition, a yaw mark may be left on the road surface by the sideward motion of a tire when the driver attempts to turn the wheel. Such a yaw mark may reveal the speed of the vehicle at the time the mark was made.

In determining speed from skid marks or yaw marks it is necessary to conduct a drag-factor test at the specific point where the tire marks were put down. This test must be conducted in the same direction that the vehicle was traveling and once that is done and the drag-factor is determined then a determination of speed can be made.

Where there is some evidence of the speed of the vehicle then a time distance analysis may be employed. That is if the speed of the vehicle is put at 25 miles per hour then the distance that the vehicle is traveling in feet per second is a function of 1.47 times that speed. In converting miles per hour to feet per second you are then able to point to where the vehicles were at various points in time.

If any object was thrown from the vehicle, then the so-called flip-vault formula may be employed to calculate the minimum speed of the colliding vehicle if the exact point of impact is known and also the exact point where the item first made contact with the ground is known. This flip-vault formula can also be used in pedestrian knockdown cases where the point of impact and the point where the pedestrian hit the ground after the impact are known.

Another formula that may be employed is the so-called fall formula which can be used where a vehicle has run off an embankment. Once the horizontal distance that the vehicle traveled before hitting the ground is determined and the vertical distance that the vehicle dropped is known then with those two numbers this formula can be used to prove whether or not the vehicle was speeding when it left the embankment.