By David Daggett

(Disclaimer: The author doesn’t claim to be now, nor has he ever been, a collision reconstructionist. While having prosecuted many cases involving the science–and having once stayed at a Holiday Inn Express–he has nonetheless always relied upon the expertise and tutelage of crash investigators when prepping for trial. This article is merely designed with two objectives in mind: (1) help familiarize prosecutors with some of the very basics of crash reconstruction; and (2) point out to crash investigators some factors that can be beneficial to prosecutors in proving the requisite degree of negligence/recklessness.)

Prosecutors cannot be expected to be experts in the field of collision reconstruction. After all, a fully-trained crash reconstructionist goes through many weeks of specialized training in order to be qualified to analyze a crash scene. Prosecutors certainly don’t have the benefit of that extensive training. When prepping for trial, however, prosecutors must develop a solid grasp of some of the more basic terms, concepts and equations used in collision reconstruction and crash investigators must be able to assist the prosecutor in developing a simple and solid presentation to the fact finder. As a team, you must answer this one question: Can we explain the facts in a clear and concise way so that 12 jurors will understand what happened?

Many Auto Manslaughter and DUI Homicide cases center around two basic issues: impairment and speed. Any competent prosecutor knows how to prove the former, so it is the latter that will be addressed here. The goal of this article is to focus on those comparatively simple terms, concepts and equations. Obviously, there are many areas of crash reconstruction (such as “vault” and “crush”) that cannot be described as “simple” and will not be a focus of this article. With that caveat in mind, the importance of having a solid grasp of the following principles cannot be overstated.

Get down to basics

There are a number of basic terms and concepts in the field of crash investigation that prosecutors must be familiar with when prepping for an Automobile Manslaughter or DUI Homicide case. The prosecutor and crash investigator should go over the facts well in advance of trial and determine which ones are applicable in your case.

Area of Impact – This is the area where the vehicles, vehicle and pedestrian, or vehicle and fixed object (tree, guard rail, pole) first came into contact. The area of impact can usually be determined by gouge parks in the pavement, oil or other fluid spills, the ending of skid marks, the location of the vehicles, property damage, witness statements or by other means.

Sight Distance – This term is used to indicate from what distance the at-fault driver could or should have first been able to perceive the hazard. Weather and lighting conditions must be taken into consideration when determining sight distance. The defendant’s height and the vehicle he was driving can also be a factor. Road topography, including hills and curves, plays a big part. This is especially important when the defendant is claiming that the victim “came out of nowhere” and that he (defendant) didn’t have time to react. Sight distance is also critical when the defendant is alleged to have passed another vehicle on a blind curve or blind hill. Passing on a double yellow line on a blind curve or hill is, in my opinion, gross negligence per se. If a driver cannot see oncoming traffic around a curve or over a hill and yet passes anyway, he is for all intents and purposes playing Russian Roulette with his automobile and that shows a wanton or willful disregard for human life.

Work with your crash investigator to conduct sight distance experiments. For example, say the defendant’s vehicle struck a pedestrian after dark on an overcast evening. To measure sight distance, attempt to re-create the scene by having a police officer of similar stature as the defendant drive a vehicle comparable to the one the defendant was driving. Have another officer, similar in size and wearing clothing comparable to what the victim was wearing, stand at or near the point of impact. Have the officer playing the role of the defendant drive in the direction of the victim, making sure to take note of when the victim first comes into sight. This will allow you to show the jury from just how far away the defendant should have been able to see the victim, thus avoiding the fatal collision. Use a dash mounted camera to record the proceedings. Another example would be when the defendant is alleged to have been speeding on a winding, narrow road. While recording with the dash cam, drive the road at both the posted speed limit and at the speed at which the defendant is alleged to have driven. This will allow the jury to see the road from the same perspective and point of view as the defendant. Be sure to have the road blocked off during this exercise, both for safety purposes as well as to prevent the defense from arguing that what the defendant is alleged to have done must not have been all that dangerous or else the police wouldn’t have done it.

Time – Distance Relationships – By determining the speed of each vehicle, the crash investigator can go back in time and re-create where the vehicles were just prior to the sequence of events leading up to the crash. This can help show whether the crash could have been avoided had the speed limit been observed and is closely related to the formula for determining distance during reaction time, which will be discussed later. Time – distance can be used to place pedestrians at particular points in the crash continuum; help determine whether a vehicle stopped at a stop sign or red light; or how long it takes one vehicle to pass another.

Critical Speed – This term refers to the greatest speed at which a vehicle can travel while still being able to safely negotiate a turn or curve in the road. At a speed exceeding the “critical speed” the tires will begin to lose traction with the road, causing it to slide off the curve due to centrifugal force, potentially leaving tire marks of some type.

Skid, yaw and gouge marks – These markings are generally found in the roadway, shoulders or adjacent roadside area and can be a telltale sign for a crash investigator.

  • Skid marks occur when a tire is locked up and not freely rolling on the surface of a road. The tires heat up and “lay rubber” across the surface of the road, so to speak. Leaving skid marks on a road surface will oftentimes leave corresponding skid patches on the tire/s.
  • Yaw marks generally occur when the tire is rotating (brakes not locked) but the vehicle is moving at too great a speed to firmly grip the road. A good example can be found when making a common turn. Generally, speed is reduced and the brakes aren’t applied yet the tires firmly hold the road. Try taking that same turn at a much greater speed and the tires begin to lose their firm grip with the surface of the road, causing them to skim over the surface, leaving yaw marks. Starsky and Hutch and Bo and Luke Duke were great proponents of this sort of driving. And the fools never wore seat belts!
  • Gouge marks are divots, scrapes or other markings left in the road and are generally caused by a metal or hard plastic object from a vehicle digging into the road surface. They are helpful in determining the initial area of impact, especially when one vehicle crosses over the center line and strikes another, and both vehicles ultimately end up far from the initial point of impact. The gouge marks are generally a good indication of where the initial strike took place.
  • Event Data Recorders – This a device in a vehicle (also commonly referred to as a Sensing and Diagnostic Module or Airbag Control Module) that records certain vehicle data during the brief period of time just prior to a crash or other dynamic event. It typically records pre-crash, crash and post-crash data such as speed, direction, steering performance, seat belt status, air bag information and other data. This information can be extremely helpful and can be used to corroborate your crash investigator’s findings. It is highly recommended that a search warrant be obtained prior to obtaining the data, or at the very least, before viewing the data, as sometimes the data will be lost if the vehicle is moved from the crash scene.

Simple Calculations, Equations & Concepts

Below are a number of different calculations, equations and concepts that can be helpful both in understanding, as well as in explaining to the fact finder, what actually occurred at the crash site. While I’ve certainly not included all possible formulae and equations, these are some that can assist you in trial presentation. Ask your crash investigator to assist you in determining which ones might be relevant to your case. While some of them may seem daunting at first glance, those covered here aren’t all that difficult and familiarity with them will go a long way towards helping to make your case.

Velocity (Feet per second) – “Velocity” means feet traveled per second. It is distinguishable from “speed”, which is usually expressed in miles per hour. Velocity is determined by a very simple formula:

Velocity =    feet in mile (5,280)/seconds in an hour (3,600)    = 1.466 feet per second

In other words, a vehicle going one mile per hour will travel 1.466 feet in one second. To determine how many feet it would travel in one second at a particular speed, simply multiply 1.466 by that speed. For example, a vehicle doing 45 miles per hour will travel 65.97 feet in one second (45 x 1.466 = 65.97 feet per second).

Perception and Reaction Time (PRT) – “Perception and reaction time” refers to the time that is expended from when a driver first perceives a potential problem to the time when the vehicle begins to respond to the driver’s evasive actions. This time frame encompasses the detection of the possible hazard, the comprehension of it being a threat, formulating a decision, responding to that decision by exerting control over the vehicle and finally, the vehicle responding to the driver’s input (for example, braking or steering.)

While many things can influence perception and reaction time, including age, health, driving experience, attentiveness, fatigue, alcohol and drugs, day or night driving, lighting and other factors, the most commonly accepted figure for perception and reaction time in the field of collision reconstruction is 1.6 seconds. Make sure that your collision reconstructionist comes to court armed with documentation verifying his/her assertion of the perception – reaction time applied in their equations.

Distance During Reaction Time (distance = velocity x time) – This equation will tell you how far a vehicle traveled during the 1.6 seconds that it took for the driver to perceive the hazard and for his vehicle to react to his actions. Sticking with the 45 MPH figure, the equation would read:

distance = velocity x time
d = (45 MPH x 1.466) x 1.6 seconds
d = 65.97 x 1.6
d = 105.55 feet traveled during 1.6 seconds at 45 MPH for an average, healthy and sober individual.

This equation becomes important when taking into consideration the effects alcohol and certain drugs and medications can have on a driver. With the proper expert testimony from a toxicologist, the fact finder will hear how alcohol and drugs can adversely affect a driver’s vision, reasoning, judgement, distance and depth perception, coordination and other motor skills, thereby affecting perception and reaction time.

If the defendant had been highly impaired and the prosecution lays the proper foundation to establish a slower than average perception and reaction time, the prosecution should be allowed (even if only with a hypothetical) to present to the fact finder comparative figures taking into consideration that delayed reaction time. For example:

Sober Driver (1.6 second PRT)                     Intoxicated Driver (2.6 seconds PRT)
d = (1.466 x 45 MPH) x 1.6 seconds            d = (1.466 x 45 MPH) x 2.6 seconds
d = 65.97 x 1.6                                                  d = 65.97 x 2.6
d = 105.55 feet travelled during                    d = 171.52 feet travelled during
the 1.6 seconds of PRT at 45 MPH              the 2.6 seconds of PRT at 45 MPH

Thus, there is a difference of 66 feet that the impaired driver’s vehicle would have travelled during that extra second of delayed Perception and Reaction Time. That is 66 additional feet that a sober driver might have used to swerve, slow down, lessen the impact or perhaps avoid the collision altogether. This is important in helping to show a causation between the defendant’s impairment, his negligence and the collision. The above example even assumes that the at-fault driver was obeying the posted speed limit, which in itself is unlikely. We’ll take a look at how speed in excess of the posted speed limit will come into play during Part 2 of this article.

To be continued in November Blog.