Challenging the Critical Speed Formula In Light Of the Daubert Decision 2005-01-3141
Yaw marks are often observed at the scene of an accident when a car spins out of control on a curve. Police investigators measure the radius of these marks and conduct skid tests at the scene to determine the friction, or drag factor, on that road surface. The radius and friction values are then plugged into the Critical Speed Formula, or CSF, to compute the car's speed. Such estimates may later form the basis of the officer's expert testimony for speed related prosecutions, including criminally negligent homicide. The CSF is derived from a more generalized equation that balances cornering force with inertia. But the CSF is too simplistic to account for all the variables that affect a cornering vehicle, and the methods normally used to employ it are not valid. By default, the braking friction value from the police cruiser is substituted into the equation for the cornering force limit of the crashed car, because cornering friction is either too dangerous or impractical to obtain at the scene. The car that caused the yaw marks may be too damaged to drive - the best evidence destroyed - and the roadway not wide enough to conduct a skid pad test. The operant hypothesis is that the braking friction limit of every test car is equal to the cornering friction limit of any car on the road.
Analysis of independent, real world data shows the potential for error using such methods exceeds 41%.