CHAPTER 24. Motor Vehicle Collision Reconstruction
Kristine Karcher
Motor vehicle collisions are considered a medicolegal event. The accurate investigation and reconstruction of motor vehicle collisions are important for determining the cause and events involved in a crash and provide valuable information and statistics that are used to produce safety changes. These include improvements in vehicle and roadway engineering, as well as information that may be useful in criminal or civil prosecutions. The annual production of vehicles continually integrates new features to improve the safety of occupants based on the information obtained. Roadway engineers will improve dangerous sections of highway based on the frequency and cause of crashes at a certain location. The importance of a thorough investigation and collision reconstruction cannot be underestimated. The forensic nurse examiner can contribute significantly in the investigation through knowledge of injuries, injury causation, and the understanding of the bio- mechanics of impact.
Laws of Motion
It is important to have an understanding of the basic physics that govern the behavior of all moving objects, including vehicles. Newton’s first law of motion states that if you are in rest you tend to remain in rest, whereas if you are in motion you tend to remain in motion, unless you are acted upon by an unbalanced external force. This describes inertia. If you are an occupant of a vehicle and it suddenly stops, the inertia of your body will tend to resist the stopping, and you will slide forward against your seat belt. Newton’s second law of motion states that the acceleration of any body is directly proportional to the force acting on the body, whereas it is inversely proportional to the mass or weight of the body. If you are an occupant in a vehicle and it is traveling 50 miles per hour (mph), your body within the vehicle will also be traveling at the same rate of speed. Newton’s third law of motion states that for every force exerted on a body by another body, there is an equal but opposite force reacting on the first body by the second. In other words, for every action there is an equal but opposite reaction. In the field of motor vehicle collision investigation, acting and reacting forces are found when a vehicle skids to a stop.
Biomechanics of Impact
The science that assists us in understanding the mechanism of injury and the tolerance of the human body is known as the biomechanics of impact and links medicine with science. In vehicle crashes, the injuries generated are related to both the speed of the vehicle and how suddenly it stops. This sudden change in velocity is known as delta V and measures the severity of the crash or impact. This change in velocity may occur to the occupant’s entire body or specific blows to certain areas. It is dependent on the varying deceleration that occurs to anatomical structures, such as the head (for example, impacting the windshield), chest (impacting the steering wheel), or lower extremities (striking the dashboard) within the vehicle. The human response to these various forces and the change in vehicle velocity contributes to the science of biomechanics (Association for the Advancement of Automotive Medicine (AAAM) 1992).
Vehicle collision investigators describe a collision as a series of four impacts:
1. When the vehicle impacts something that changes or stops its speed
2. When the occupant is thrust toward the direction of the impact, striking the interior of the vehicle
3. When the occupant’s various body parts collide within the person’s body. (Besant-Matthews, 1998)
4. When objects (pets, groceries, aluminum cans, etc.) being carried within the vehicle impact the body.
The human body, observing the laws of motion, continues to move forward at the same rate of speed that the vehicle was traveling. The shorter the time and distance to stop, the greater the force required to bring motion to a halt. The opposite is true when the time and distance to stop deceleration is delayed—the force is reduced. Consequently, if a person decelerates more gradually, this will improve the chance that less bodily injury will occur (Besant-Matthews, 1998). The slower the deceleration, sometimes by just a few feet or a fraction of a second, the greater chance there is for positive occupant outcome.
Automotive Engineering Improvements
Automotive engineering improvements include seat belts, air bags, auto safety glass, and changes in dashboards.
Seat belts
The development and implementation of seat belt use has proved the most effective advancement in the prevention of serious injuries and fatalities. Seat belts were designed to provide a longer ride down (deceleration) within the vehicle, allowing occupants to avoid impact with the interior of the car. The longer the ride down, the less chance there is of injury. This is accomplished by the actual stretching of the seat belt, as much as 6% to 14%, to allow the occupant to decelerate slower and ride down the collision. Seat belts also prevent ejection from the vehicle by holding the occupants in their seats and distributing the force more evenly over the body surfaces (Besant-Matthews, 1998).
The force in a serious crash is such that parts of the human body are still going to contact the steering wheel, dashboard, or windshield. The average forward motion in an abrupt collision at 34.2 mph can result in forward movement of the head 22 inches, forward movement of the chest 153/4; inches, and forward movement of the pelvis 141/2 inches. This forward motion is due to seat belt stretch, seat belt slack, improper use of the seat belt (i.e., wearing it too loose or in the wrong position) (Besant-Matthews, 1998), and, finally, momentary flexing of vehicle parts. The ride down of properly worn seat belts has reduced death and injury by 45% (AAAM, 1992) (Fig. 24-1).
Injuries associated with seat belt use range from superficial abrasions in minor collisions to major internal injuries in high-impact collisions. Lap belts may cause lacerations or injury to the liver, spleen, omentum, and mesentery, whereas shoulder belts are often the cause of injuries such as fractures to the spine, clavicle, sternum, and rib cage.
Air bags
To improve and supplement seat belts, air bags were developed. Not only do they protect the chest from the steering wheel and column, they provide gradual deceleration of the head and neck, preventing the whiplash motion of frontal impact. The use of air bags has significantly decreased injuries to the neck, face, and head that only a few years ago were commonly seen when drivers impacted steering wheels. As with seat belts, air bags also have associated injuries (see Chapter 23).
Commonly made of a nylon-type material, air bags are kept folded in a container until sensors detect a frontal collision of sufficient force, causing them to deploy. Air bags deploy at speeds of up to 150 to 200 mph in 24 to 45 milliseconds. Bursting through the flaps that cover them, air bags deploy with enough force to dislocate, fracture, and even amputate thumbs (Spitz, 2006). They are capable of causing an array of injuries including fractures to the forearms, abrasions and contusions to the face, corneal abrasions, and head injuries (Spitz, 2006). With the increased use of side air bags, related injuries including rib fractures and related contusions and abrasions will be observed. In spite of associated injuries, air bags and seat belts have played a major role in decreasing the fatalities associated with motor vehicle collisions.
Auto safety glass
Another safety improvement to vehicles is to the material content of windshields. They are made of two layers of glass, separated by a thicker interlayer of polyvinyl butyryl. This allows the windshield to absorb more energy and freely bulge (as much as 5 inches) before the plastic interlayer tears. In a high-speed collision, the body is thrust forward and the head arches forward and downward, making contact with the windshield and often resulting in large, deep cuts. These cuts may be horizontal, diagonal, or parallel to the face and neck because of head bobbing after impact. In less severe impacts, the incisions to the face will be more superficial. These injuries can be seen on the nose and forehead and result as the head slides down the sharp edges of broken glass (Spitz, 2006) (Fig. 24-2).
Side windows differ in construction and are not laminated, as are the windshields. Injuries associated with impact to the side windows are very identifiable. When side glass is broken, it disintegrates into numerous 1/4-inch cubes with sharp edges. These produce right-angled superficial cuts to the skin and are referred to as dicing injuries (Spitz, 2006). Dicing injuries are easily recognized and are usually located on the side of the face, shoulder, or arm nearest the involved side window. The presence of dicing injuries can assist in determining where an occupant was seated in the vehicle at the time of impact. In the clinical setting (hospital, emergency department), pieces of glass that remain embedded in the wound would be secured as evidence (Figs. 24-3a and b).
Dashboards
Change of dashboard contour is another safety feature development that is worth discussion. Dashboards have the capability of changing form up to 3 inches, contributing to a slower deceleration. The knees commonly contact the dashboard in frontal, head-on collisions. The severity of injuries to the knees depends on the amount of contact that occurs with the dash and may range from a bruise or minor abrasion to a large laceration or dislocation/fracture. It is important to examine both knees for even subtle injury. The steering wheel column has undergone safety features as well and is constructed to collapse when impacted by an occupant, allowing for a slower ride down.
Probably the most overlooked piece of evidence on the body at a collision scene is patterned injuries. These usually arise from contact with something within the vehicle, such as a knob or handle, and may be subtle or very impressive. As with other injuries, they may help investigators determine the position of the occupant, as well as the direction of impact.
Collisions
Types of collisions to consider include frontal, side-angled, motorcycle, and automobile versus pedestrian crashes.
Frontal
Frontal collisions account for 50% of all motor vehicle crashes. The force to a unrestrained occupant in a frontal crash of a vehicle traveling 30 mph and impacting a rigid object resulting in a 2-foot intrusion to the bumper is equivalent to falling out a third-story window onto concrete. This intrusion to the front of the vehicle occurs in 90 to 100 milliseconds or one tenth of the time it takes to blink (AAAM, 1992). Vehicles do not have structures that crush uniformly. Front structures such as the bumper or grill are much less rigid and do not actually slow a vehicle as much as contact with more rigid structures such as the vehicle frame, engine, or suspension. As previously mentioned, an unrestrained front seat occupant of a vehicle traveling 30 mph would strike the dashboard, steering wheel, or windshield with different parts of the body. The injuries would then relate to the structures within the front of the vehicle that were impacted and the amount of localized deceleration of each. For instance, was the occupant’s head experiencing a slower ride down because of windshield bulge, or were chest injuries lessened because the steering wheel column collapsed?
Side or angled
Side or angled collisions account for approximately 20% of motor vehicle crashes (Fig. 24-4). The occupants move toward the point of impact and often make contact with the windows, A-pillars (roof supporters between the windshield and front door), or roof. Even in 90-degree side impacts, lap belts will restrain the occupant. The rotation of vehicles after impact will often complicate the injuries observed to the occupant. Common injuries that result from lateral or side impacts include rib fractures, lung contusions, hemothorax, pericardial damage, and aortic rupture or tears. If the vehicle rolls over and makes no contact with a solid object, the injuries observed may be minor, but if the vehicle strikes a solid object, the injuries will be more severe. Ejection of occupants through the doors or windows of a vehicle are not uncommon in rollovers. Ejected occupants often have severe neck, head, or brain injury, and research has shown that most injuries to occupants are sustained before they are ejected.
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