Forensics
- Forensic Evidence
- Preserving the Crime Scene
- Autopsy
- Interview Witnesses
- Coercing Confessions
- False Confessions
- Fingerprint Evidence
- Gunshot Wounds
- Child Homicide
- Drowning
- DNA Evidence
- Accident Reconstruction
- Forensic Botany
- Forensic Dentistry
- Trace Evidence
- Digital Evidence
- Forensic Photography
- Collecting Evidence
- Toxicology
Accident Reconstruction : Defending the Accused
Motor Vehicle Homicide: Investigative Reports
If you are charged with motor vehicle homicide, your criminal defense attorney should have a thorough understanding of accident reconstruction investigation, theories, and physics. Accident reconstruction “experts,” like all other policemen, draft police reports. These reports are of varying quality. Some reports are so poorly drafted and include so many factual inaccuracies and misapplied theories that it is difficult to believe that the reconstructionist is permitted to pawn himself off as an expert. At a minimum, the collision report should include a list of vehicle damage, a checklist for standard classifications of the vehicle, a diagram or outline of the vehicle with the damaged areas marked, a diagram of the area with code numbers that can be referred to, an estimated dollar value of the damage, and coding from a damage scale manual.
Damage to Vehicles
When dealing with minor collisions, the investigator only needs the information for the collision report form, such as vehicle identification and the owner’s information. Severe collisions, on the other hand, require more effort on the investigator’s part. He must first prioritize and gather whatever information he can that may be lost if not collected quickly. He also needs to take notes on the final position of the vehicle, whether there are lamps on after dark, any major parts that are detached, the condition of the tires and wheels, whether or not the steering wheel is locked, whether or not the vehicle was carrying cargo, the presence of alcoholic beverage containers, the speedometer reading, and whether or not seatbelts were used. He also should take photographs. There are many ways of classifying the damage incurred by vehicles in a crash. Contact damage is “damage to any part of a motor vehicle by direct contact with some object which is not part of the vehicle.” (Baker, 85). Contact damage is further broken down into internal and external contact. Internal contact damage is caused by the occupants or anything else that was in the vehicle. External contact damage is cause by anything outside that touches the vehicle.
Induced damage is defined as “damage to any part of a motor vehicle caused by some other part of the same vehicle or by the shock of collision.” (Baker, 86). This occurs when parts get pushed out of position, which may also damage other parts in the process.
Distinguishing between contact and induced damage can assist the investigator in determining the exact position of the vehicles when the collision occurred, as well as whether or not there was more than one collision. The investigator can tell if there was more than one collision by undamaged areas between contact damage. This often occurs when two cars are driving parallel, and they hit in the front, then spin outwards and hit again in the back.
Imprints and rub-off are other examples of vehicle damage. Imprints are dents that show the shape of what was pressed into the vehicle. However, these usually occur on areas that are damaged or collapsed, so the investigator cannot make out what it was that hit the vehicle. Rub-off is material that is transferred to the vehicle from whatever hits it, such as paint, rubber from tires, or skin, blood, or clothing from a pedestrian. Rubber rub-off from tires is difficult to recognize because it looks like dirt, but it is helpful in identifying which tire hit which part of the vehicle, and therefore how the collision occurred.
Imprints and rub-off indicate whether the vehicles were in motion during the contact. If the vehicles were not moving, the imprints and rub-off would be clear and not smeared. This is known as full-impact. If the vehicles were not standing still and were moving at different speeds, it would result in smeared imprints. This is known as partial impact.
Determining Force
The damage to the vehicle can be used to determine the principal force applied during the collision and from what direction it originated. However, determining force can be confusing. Too many investigators assume that the force was centered where the contact damage caused a collapse, but this is not always true.
If both vehicles are moving, the forces against them are always opposite in direction. However, the directions depend on such factors as weight, speed, and direction of travel. It is very easy for the investigator to make mistakes when judging the direction of force, especially if he is using only photographs. The direction in which occupants move is generally the opposite of the direction of the force. You cannot use their positions as a source, however, if the vehicle rotates rapidly during the collision.
Other Factors in Accidents
Evidence of a roll-over includes dirt and abrasion marks on the roof or hood, a collapse of the roof, windshield frame, or trunk lid, and scratches on the side and top of the vehicle. This information can usually tell an investigator how many times it rolled over. A vehicle rarely turns more than once unless it is rolling down a steep bank. Therefore, an experienced investigator will dismiss statements from witnesses or occupants claiming that the vehicle flipped many times as unreliable.
To determine which occupant was driving, an investigator normally compares the injuries of each occupant to those he would expect someone to have based on their position in the car. From the location of blood, hair, and clothing, it is sometimes possible to prove where a passenger was seated.
The condition of the vehicle is also useful in investigating an accident involving a pedestrian. If the pedestrian is hit head-on, there may be headlight and grill breakage, dents in the hood, and signs of scraping or breaking. If the pedestrian is sideswiped, his buttons and clothes may leave brush marks on the vehicle’s paint. If backed into, the pedestrian’s blood, clothing, or tissue may be found on the bumper, trunk, and license plate. Finally, if run over, the pedestrian’s blood and clothing may be left on the underside of the car, motor, frame, and wheel.
Road Examination
In accident reconstruction, there is nothing more important than the investigator’s proper and thorough examination of the roadway. Tire marks that are left on the ground after an accident can be divided into tire friction marks, which are made when the tire rubs the road due to slipping or sliding, and imprints, which are made from a rolling tire that did not slip. Examples of tire friction marks are skidmarks and scuffmarks. Skidmarks are made by a tire that is sliding without rotation, and scuffmarks are made when a tire is both sliding and rotating at the same time.
Skidmarks vary in appearance based on the surface of the road. The length of time that they last depends on the kind of mark, the weather, the amount of traffic, road repairs or reconstruction, and special characteristics of the tires. Marks on a wet surface may last a minute or less if it is raining or if many vehicles are passing over it. However, dark friction marks or deep furrows can be visible for as much as a year after the incident.
Skidmarks can tell an investigator how and where a collision occurred. If properly understood, they can provide information vital to estimating after-collision speeds. However, they are not always easily understood. The exact point where skidding begins is difficult to determine because the tires might not leave a visible mark. The driver might have to travel some distance before the friction gets hot enough to cause skidmarks. Skidmarks are not usually caused by the tire leaving behind rubber, but instead by the heat and friction of the tire scarring the roadway.
A yawmark is the most important form of scuffmark. It is made when a rotating tire slips parallel to its axis. By measuring a yawmark, a pre-collision or post-collision speed can be estimated. Yawmarks are different from skidmarks because they are made by steering rather than by braking. On a dry surface, a strong yawmark may only last three months, while on a wet surface, it may disappear within three minutes. It is also possible for a yaw to leave no mark. Unfortunately, many investigators mistake after-collision skidmarks for after-collision yawmarks. Such a mistake will lead an investigator to make a wildly inaccurate pre-collision speed estimate.
How the Accident Occurred
Accident reconstructionists may make several important mistakes while investigating a single accident. Just one mistake can invalidate an investigator’s pre-collision speed estimate. Investigators sometimes determine the positions of the vehicles or pedestrians at the start of the collision by carelessly relying on a so-called witness or on someone who received information from someone who might have seen the accident. Many investigators improperly photograph the scene, the scuff marks, and the vehicles. Despite the need for accuracy, some use little care in measuring scuff marks. In fact, it is not unusual for an investigator to simply guess at a distance. Photographs can also be misleading and are easy to misread. Investigators, sometimes, are unaware that the post-collision resting positions of the vehicles and vehicle debris have been moved prior to the investigator’s arrival. Poorly trained investigators mistake extraneous marks on the highway or street for marks caused by the accident under investigation.
To determine the first contact point, the investigator may examine skidmarks or yaw marks. These marks, if properly understood, will yield an enormous wealth of information about the accident. A poorly trained investigator may look at the positioning of the debris to determine where the accident occurred. Unfortunately, using the debris to estimate the original point of contact is a waste of time. How the debris scatters will be affected by: (1) the speed of the vehicles at the time of the collision; (2) the path and rotation of the vehicles following the collision; (3) other vehicles traveling through the debris field following the accident; and, (4) the wind.
By measuring the damage to a vehicle, the force and velocity of the impact can be accurately assessed. However, the damage does not by itself tell you which vehicle was moving faster in the collision. The stronger/heavier vehicle, even if it was going faster or slower than the other vehicle, usually suffers less damage. Speed is not determined by damage but rather by estimating the after-impact velocity of each vehicle and applying the principles of conservation of momentum. To try to determine how the accident occurred, the investigator must also draw a to-scale diagram of the vehicles and specify exactly where and to what extent they were damaged. By doing this, the investigator can figure out the behavior of the vehicles in the accident.
By combining the information he has learned by inspecting the damage to the vehicles, the length and curvature of any scuffmarks, and any witness statements, a competent investigator should be able to determine how an accident occurred and assess blame, if any. But every piece of verifiable information must fit within this theory. If the theory fails to take into account every skidmark, scuffmark, and vehicle damage, it is unreliable.
Kevin J. Mahoney is a Massachusetts criminal defense attorney and author of Relentless Criminal Cross-Examination. He has successfully defended those accused of motor vehicle homicide.
References
Baker, Kenneth S. Traffic Collision Investigation, Northwestern University, Evanston, IL, 2001.
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