Many criminal events can be looked at as hazards, accidents, or at the very least, caused occurrences. Someone is doing the wrong thing in the wrong place at the wrong time, and there are elements of human failure, operator error, mistake, negligence, and intent, just to name a few. With accidents, for example, courts have come to rely upon experts to determine pre-existing conditions, risk and hazard assessment, and causes. A tradition has existed with the use of such experts in civil liability cases (tort law) for many years, and in recent years, there has been a profound increase of such use in criminal law.

Although traffic accidents are the most common area of application, the expertise is by no means limited to just that. It is helpful to distinguish between two (2) types of experts: (1) investigators -- who try to determine the causes of an event; and (2) reconstructionists -- who try to mimic conditions before the event took place. A few investigators are reconstructionists, but not many, and courts have broad discretion in whom they will consider a forensic reconstructionist. Police investigators must generally have some advanced training, extensive experience, and membership in professional associations. Any person proffered as an expert will usually be probed before the court in a procedure known as voir dire of the expert. This will then determine the expert's competency to testify. Procedures are fairly rigorous because if the trial court too easily accepts or rejects such an expert, this is grounds for reversible error and the overturning of a verdict at the appellate level. The following are standard grounds for qualification:

Experts don't have to be mechanical engineers, but it helps to have things like a bachelor's and master's degree in mechanical engineering with a thesis concentration in biomechanics. An associate's or bachelor's degree is more common. In other cases, years of experience as a police investigator who has attended numerous seminars and been recognized before by courts as an expert may be all that's necessary to qualify. You can't put an arbitrary number on it -- some jurisdictions will require having testified six (6) times before, others less or more. The accepted laws of motion and mechanics form the basis of this expertise, which are easily learned in a variety of domains: aerodynamics; hobbyists; manufacturing; safety; transportation, etc. Again, courts have wide discretion, and will often limit the testimony of the expert to one or two points, such as safety, but not design issues. In addition, some states, but not all, restrict the expert from testifying as to the ultimate issue -- an opinion or inference as to cause and effect which embraces the issue of who is at fault or guilty. The ultimate issue is usually the province of the jury, and courts that allow the expert to testify about it leave it to the jury to weigh both the expert's credibility and their opinion on the ultimate issue.

Most accident investigators are trained on the job, on a one-on-one basis through actual field experience. It's a difficult career climb from traffic patrol officer. They must arrive quickly at the crime scene and consider each situation on its own merits, not follow some standard checklist. They must collect evidence, take witness statements, and determine the cause. Evidence as to the cause will lead to the cause. Although taking statements and ascertaining the condition (intoxication) of witnesses are done, probably more important is determining the working condition of the vehicle involved -- brakes, lights, wipers, turn signals, mirrors, etc. Then, the surrounding physical conditions are noted -- roadway surfaces, loose material, weather, visibility, etc. All observations must be made from the driver's line of sight. Photographs, sketches, and accident reports are then filed routinely.

Crash analysis consists of steps not unlike those taken by a coroner or forensic pathologist. First of all, the event is classified into one of three key events and one of five crucial events:

KEY EVENTS:	CRUCIAL EVENTS:
running off the road noncollision on the road collision on the road	leaving the available path turning over in the path without collision other noncollision in the path collision with a nontraffic object collision with another traffic object

DIRECT: determined by finding the "point of no escape" which usually means excessive speed, failure to stop, crossing a line, or some statutorily defined criminal offense.

MEDIATE: determined by finding the collision course which usually means circumstances beyond the person's control such as weather, faulty equipment, or an angry driver.

REMOTE: determined by background checks and include driving habits, roadway design, warning signs, and vehicle maintenance history.

In cases of multiple accidents at the same scene, or catastrophic accidents (like those involving aircraft and federal investigations), the rule of first harmful event is followed which means that the accident which occurred first is the one that gets classified. Special procedures exist, of course, with airplane crashes as they do with commercial trucks and other vehicles often requiring a HAZMAT team.

This is an area of expertise in which the expert must not only know various laws of physics but also how to explain mathematical formulas. When faced with an emergency, most drivers apply the brakes and/or swerve their vehicle. The resultant skid marks provide a means by which the minimum initial speed can be determined. Note that the previous sentence said "minimum" speed only because skidmark analysis almost always underestimates the actual speed to the benefit of a speeding driver.

Skidmarks are measured from the terminal point backwards. Marks from the front wheels must be distinguished from marks by the rear wheels. If they overlap, divide by the wheelbase. Each wheel mark, should in fact, be measured separately, if possible. On a curve, the tape measure should follow the curvature. It is possible for curved skidmarks to be what are called yaw marks. Yaw marks occur when a tire tries to rotate in one direction while sliding in another. Yaw marks can also yield information about speed, direction of travel, and point of impact. Skidmarks are dark and have clear beginnings and ends, as opposed to tire imprints and scuff marks which tend to show tread or stipples and are not as affected by driver actions as skidmarks.

Marks occur because small particles of rubber are torn off the tires and internal tire temperatures average between 200 and 1000 degrees Fahrenheit. The principle is one of kinetic energy -- energy varies as the square of speed -- where foot-pounds (kinetic energy) are transformed by sliding into heat energy (stopping energy). By the law of conservation (of energy), energy cannot be created or destroyed, so the energy of motion must be equal to the energy of stopping.

Another principle is that of centrifugal force, which is used when there is an impact, and holds regardless if the brakes have been locked or not. Centrifugal force always resists the direction the operator is trying to control. It produces skidmarks which are less visible and often curved. These type of tire marks are called impending, because the wheels are just about to stop rotating. By contrast, locked marks are usually straight and left by a nonrotating wheel. It's important to collect information about both impending and locked wheel skids.

There are about 22 different mathematical formulas involved with skidmark analysis, but we'll only deal with two (2) -- the Coefficient of Friction and the Basic Speed Skid Formula:

The coefficient of friction (f) is the ratio of force needed to slide an object (F) compared to the force an object exerts on a surface (W). The coefficient of friction varies slightly with how wet the surface is, so it, and all subsequent calculations are best when confined to dry surfaces, and most roadway surfaces require 3000 pounds of force to slide something on them. The coefficient of friction (also called the drag factor) represents the force necessary (usually 3000 pounds) to move the weight of a vehicle (usually 5000 pounds). It's common to arbitrarily assign a coefficient of friction of 60% as in the example given.

In the basic speed skid formula, S is the skid distance, V is the velocity or mph, and f the coefficient of friction. S is easily measured and f is known, so all that's left is to work the formula (transposing results in V equal to the square root of 30Sf). Again, this results in a fairly conservative estimate of the minimum speed of the vehicle, but it has held up in court well. Some skidmark experts will go further and conduct their own test skids in close cases.

When known facts are entered into a computer, the computer can then generate conclusions based upon assumptions in the software being used. Animations are one form of forensic reconstruction. Other forensic experts have been doing reconstructions for years, to try to figure out what a disappeared person would look like after 10 years, for example. Forensic animators, however, use animation and computer-aided design (CAD) software to make images of auto crashes and crime scenes. Their creations try to answer questions like: what was the driver's view on the foggy night or what the witness could and couldn't see from a third-story window.

Sometimes, the testimony is a frame-by-frame demonstration. Other times it's a full motion video simulation. Simulation (expert testimony) is given greater scrutiny by the court than demonstration (adjuncts to testimony). A simulation often involves the mathematical capability to project different possible outcomes of what happened.

Forensic computer animations are subject to strict discovery and disclosure rules. Under Frye standards, it must be determined what field of science is involved: physics, engineering, or accident reconstruction. Under Daubert standards, calculation accuracy/error must be computed. It must also be determined that probative value outweighs prejudicial effect. Experts must be able to explain the factual basis for the animation (sometimes the software's algorithms), the kind of computer they're using, where they received their information, and the expert numbers they were given. In many ways, the credibility of the reconstructionist depends upon the credibility of the investigator.

PRINTED RESOURCES
Hand, B., A. Sherman, M. Cavanagh. (1980). Traffic Investigation and Control. NY: Macmillan.
Moenssens, A., J. Starrs, C. Henderson & F. Inbau (1995) Scientific Evidence in Civil and Criminal Cases. Westbury, NY: Foundation Press.