Banner for The Criminal Justice Systems: A Guide for Law Enforcement Officers and Expert Witnesses in Impaired Driving Cases

Toxicologists, Crash Reconstructionists,
Optometrists, and Other Medical Personnel


The physics and math used by crash reconstructionists can be some of the most confusing testimony any prosecutor will have to elicit and any juror will have to hear. Even before trial, a prosecutor handling a crash case that resulted in injuries or death will need to meet with the crash reconstructionist to learn what the police report means—Who was driving? What role did the driver(s) play in the crash? Were the EDRs (“Event Data Recorders” commonly referred to as “black boxes”) recovered? If yes, what information did they yield? If no, get them to find out what information they contain. (This may require a search warrant.) Were there mechanical problems with any of the involved vehicles? If yes, should the owner have been aware of these problems? Have there been any recalls for manufacturer’s defects? Was road design/condition a contributing factor? Were traffic lights working properly? What was the weather like? What were the lighting conditions?  What evidence has been secured to document the details (photos of the scene, the vehicles, and the victims; scale drawings; photographing in place and then securing crash debris)?

The variables are many and all need to be discussed between the police, prosecutor, and crash reconstructionist to determine appropriate criminal charges, if any, and how to proceed at trial.

If the case warrants criminal charges, what can the crash reconstructionist expect at trial? He or she will have to discuss the pre-impact, impact/engagement, and post-impact—the moments just before the crash, the dynamics of the crash, and the immediate results after the crash.6

At trial, after having gone through the general areas of inquiry for all expert witnesses (see above) and possibly having been questioned by the defense attorney, the prosecutor will ask you to explain what you did in this case:

  • You were contacted by the prosecutor?
  • Did you review the police report, including photographs, scale drawings, and statements?
  • Did you visit the scene (evidence still present or no longer present)?
  • Did you do scale drawings/calculations using accepted math formulas?
  • What were the results of those calculations?

Notice what’s missing from that list? There is no need to go into great detail about specific calculations you did or the math and science of crash reconstruction, at least not when answering the prosecutor’s questions. However, be prepared to explain on cross examination—the questions asked by the defense attorney after you’ve answered the prosecutor’s questions—the calculations you used and the laws of physics you relied on to reach your conclusions.


6 For further details, see APRI’s Crash Reconstruction Basics for Prosecutors authored by John Kwasnoski and available for downloading at