Since the 1930s, physicians have been integral in recognizing patterns of injuries that result from motor vehicle accident and have suggested design changes that, once adapted, led to fewer injuries and fatalities. That role has reduced over the years. However, the modern day forensic pathologist, along with his or her clinical trauma service colleagues, plays a related role in identifying patterns of injuries and mechanisms of death that can aid in understanding the circumstances of a motor vehicle collision event.
Properly interpreting injury patterns can provide useful information for accident reconstruction. The injuries to the body can be the equivalent of a statement from the only unbiased witness to the accident. Information the pathologist provides helps law enforcement, attorneys, and surviving loved ones understand what happened, where the occupants were seated, how quickly they died, and potential causes for the accident.
Mary Ward, a scientist in Ireland in the 1800s, is credited with being the first motor vehicle accident fatality. In 1869, she fell under the wheels of an experimental steam car built by her cousins. 
Charles and Frank Duryea put an engine into a modified horse-drawn buggy in 1893 in Springfield, Massachusetts. The next year, a French company designed the first vehicle that, unlike the motor carriage of Massachusetts, truly resembled the vehicles of today, complete with a passenger compartment and an area where a driver could sit behind a steering wheel, brake, and clutch. 
Throughout the 1930s, physicians recognized the trauma related to automobile crashes and concluded that relatively simple modifications such as seat belts and padded dashboards could drastically reduce injuries and fatalities. The automotive industry was reluctant to add safety devices reasoning these would: 1) detract from the style of the vehicle, 2) cost too much to design and install, and 3) create the irrational fear that because safety features were needed, the vehicles were unsafe. 
It was not until 1966 -- 74 years after the combine engine was put into the buggy -- that Congress mandated installation of seat belts. However, another 18 years passed before New York became the first state to require seat-belt use in 1984.
In 1987, airbags were required as standard, and in 1991, a federal mandate required that all cars manufactured after September 1, 1997, have both driver and passenger air bags. 
In 2006, motor vehicle crashes were the leading cause of death for every age between 3 and 34 years. On average in 2008 in the United States, a person died in a motor vehicle crash every 14 minutes, and a pedestrian died every 2 hours. In the same year, motor vehicle crashes were the leading cause of death for children aged 3-14 years, and 16% of those children were killed in alcohol-impaired driving crashes. Motor vehicle fatalities and injuries have declined over the last 10 years, whether measured by population, registered motor vehicles, or vehicle miles traveled. [6, 7, 8, 9] However, in 2012 there was a slight increase reported. 
Pedestrian fatalities have also declined over the same time period, including a 50% reduction in child fatalities.  However, motorcycle fatalities and injury rates have increased over the last 10 years, whether measured against number of registered motorcycles or miles traveled. 
Alcohol impairment was involved in 32% of all motor vehicle fatalities -- a mere 6% decrease since 1998. An average of one alcohol-related fatality occurs every 45 minutes, and drivers between 21 and 24 years old are those who have greater than a 0.08 g/dL blood alcohol concentration [BAC] compared with other age groups. 
Alcohol involvement was reported in either the driver or the pedestrian in approximately 50% of pedestrian fatalities, although it was seen more frequently in the pedestrians.  In addition, almost 50% of the motorcyclists who died in single-vehicle crashes in 2008 had blood alcohol concentrations of 0.08 g/dL or higher. 
Safety-restraint use increased from about one half of all occupants in fatal crashes in 1998 to two thirds in 2008. Still, one third of occupants in fatal crashes did not use restraints. 
Overview of the entity
Injuries and issues in motor vehicle fatalities can vary widely. Drivers, passengers, adults, children, pedestrians, and motorcycle riders present with a range of patterns and types of injuries. The autopsy, accompanied by a thorough and accurate report, adequate photographic and diagrammatic documentation, and appropriate toxicologic testing plays an integral role in determining the sequence of events that led up to the accident and, ultimately, the death. Paying attention to recurring patterns of injuries can also potentially lead to improvements in automobile design and safety equipment.
Indications for the procedure
Involving the coroner's or medical examiner's office is essential (and, of course, mandated) in the investigation of a motor vehicle-related fatality. Some may ask, "The cause of death is obvious; why bother doing an autopsy?" The question assumes 2 things incorrectly: 1) the only useful information obtainable in an autopsy is the cause of death, and 2) the cause of death is the injuries.
In truth, the cause of death is nearly always related to the injuries. However, determining if the blunt trauma or the fire caused the death in a vehicle fire can be an important distinction. Discovering previously undiagnosed natural disease can provide information as to what factors may have played a role in the cause of the accident. Toxicology data are essential to any investigation, and often, these can only be obtained during a complete autopsy. If no autopsy is performed, room is created for speculation. When an autopsy is performed, it should be complete, including examination of the chest and abdomen, head, neck, and extremities, when indicated. Partial autopsies give partial answers.
Although patterns of injuries may appear obvious on the skin, and they are clearly important, examination of the distribution of fractures and visceral injuries may be the only way to understand the events during the accident. For example, internal examination of the legs of pedestrians is the gold standard when interpreting bumper impact injuries.
Motorcyclists often wear helmets and full-body, thick leather, and other protective gear that can minimize or even eliminate external injuries. Examination of the internal injuries may be the only means of documenting the mechanics of their death.
Dicing: These are short, linear, angulated, incised injuries caused by the characteristic cubed fragments of shattered tempered glass (see the image below). Tempered glass is found in the side windows of automobiles and not in the windshield or rear window. Because there is often an impact of the skin against the flat, tempered, glass window as it shatters, the incised wounds can also have an abraded component.
Stretch lacerations: These are superficial, serpiginous to linear, parallel tears in the skin in areas of excess stretching, as shown in the image below. Stretch lacerations are most commonly seen in the inguinal region of pedestrians when they are struck from the rear. These lesions usually exhibit minimal to no bleeding, but they can flank either side of larger, deep lacerations.
Avulsion pockets: When a bumper impacts the lower leg of a pedestrian, the force can shear the skin and subcutaneous fat away from the muscles, resulting in a hemorrhagic space. These pockets are a good clue as to the impact site and, therefore, the direction from which the impact came. When there is accompanying laceration in the skin, trace material from the automobile may be embedded in the pocket. Avulsions can also occur on the scalp when a tire runs over the head.
Bumper fracture : This is a fracture that occurs when the leg is struck by a bumper, and it is most often seen in the lower leg. At higher speeds, a characteristic fracture (originally called a Messerer fracture [12, 13] ) can be produced. This fracture is wedge shaped, with the apex of the wedge pointing in the direction of force, whereas the base represents the direction from which the force came. See the following image.
It is not often practical for forensic pathologists to go the scene of a motor vehicle fatality. However, when possible, viewing the scene first hand can provide vital clues about the dynamics of the accident and the source of the injuries. If the forensic pathologist cannot personally go to the scene, thorough documentation by a well-trained representative can be a close substitute. Obtaining the law enforcement agency's accident report will also be helpful. (Scene investigation will be discussed in a separate article.)
Examination of the vehicle can also be done at the impound area some days after the accident. Looking for blood, tissue, or hair can reveal impact sites. Distortions of the steering wheel, gear shift, etc, or examination of the knobs, dials, and vents on the dashboard may help explain patterned injuries. See an example in the image below.
In pedestrian hit-and-run accidents, information that can be gleaned from postmortem examination may be critical in the accident investigation. Forensic examination of paint chips, fragments of the offending vehicle, or tire tread marks can provide vital leads. Your examination must include the victims' clothing, because vehicle material or tire marks may be present on the clothing but not on the body. Obviously, examining the body may yield these clues as well. If an avulsion pocket in the lower leg lacerates and opens, foreign material may get embedded within the wound. Such material should be collected and transferred to law enforcement or appropriate crime laboratory personnel.
Collection of hair that may be embedded in a windshield can help identify an individual as the driver versus a passenger or implicate a vehicle in a hit-and-run accident.
Gross Examination and Findings
When discussing motor vehicle-related fatalities, it is useful to divide the discussion between vehicle occupants and pedestrians. Of the occupants, distinctions between the driver and passenger, use of safety-restraint devices, type of accident (eg, rollover vs head-on collision), and whether or not they were ejected should be made. Special mention should also be made of motorcyclists. Regarding pedestrians, the distinction between adults and children and the type of vehicle are important to consider, as these factors have implications regarding the types of injuries that will be seen.
Occupants of the vehicle are injured by "second impact" within the passenger compartment, intrusion of portions of the vehicle into the passenger compartment, external objects entering the passenger compartment, mechanical or positional asphyxia, fire, or ejection from the vehicle.
The "second impact" follows the actual collision of the vehicle, and it is the act of the occupant continuing to move while the vehicle is slowing and impacting the inner surfaces of the car. Even at city speeds, the accident duration (the time from the initial contact to the point when the vehicle has stopped its forward momentum) is generally 1/10th of a second. That may seem fast, but if the occupant continues moving forward within the vehicle and strikes the dashboard or windshield, the duration of time it takes for them to slow is around 1/50th of a second.
Slowing the body with the vehicle is the best way to survive an accident, as the vehicle is designed to absorb part of the impact. Seat belts keep the body strapped to the vehicle and control the deceleration of the body. Although these restraints cannot prevent all deaths, they have proven extremely effective in reducing the chance of fatalities. The National Highway Traffic Safety Administration (NHTSA) estimates that seatbelts have saved 255,115 passenger occupants over 5 years of age between 1975 and 2008; 13,250 of those were in 2008. 
Blunt force trauma is sustained by drivers and passengers when they impact the interior surfaces of the vehicle. Abrasions, contusions, and lacerations are often extensive, but in belted occupants, the back is generally spared. Patterned injuries can sometimes be seen, and when coupled with a thorough scene investigation, can reveal the position of the individual in the vehicle.
Modern vehicles have tempered glass -- specially heat-treated glass designed to break into small cubes on impact -- in the side windows. When the occupant impacts and breaks these windows, or an impact breaks the glass and propels the fragments, characteristic marks are left on the skin. These marks are generally linear or angulated, and approximately 1/4 are superficial injuries, as shown in the image below. Although they are glass injuries and largely incised wounds, these wounds typically also have an abraded component, because the skin may impact the flat window before its shattering.
Based on where on the body these injuries are, one can make determinations about position (left vs right) in the vehicle. However, glass can travel across the inside of the vehicle, such that a right-sided "T-bone" vehicle impact can send tempered glass fragments across and injure the right side of the driver's face. Dicing fragments will tend to spread with distance, similar to shotgun pellets.
If a vehicle is not equipped with air bags, or the air bag did not deploy in the accident, patterned injuries corresponding to the steering wheel can be seen. Impact with the steering wheel also results in extensive internal thoracic injuries.
The windshield is the first thing the passenger will impact  ; drivers will also impact the windshield, but after hitting the steering wheel. The injury is usually a group of vertically oriented abrasions on the forehead. These can be superficial or deep, and lacerations may also occur. Modern windshields are laminated -- 2 layers of glass separated by a layer of polyvinyl butyral (PVB), a type of resin. This prevents vehicle occupants from being ejected through a shattered windshield or prevents the occupant's head and neck from sustaining extensive injuries while bouncing around in the defect in the glass.
Thoracic injuries include rib and sternum fractures, spine fractures, pulmonary contusions, rib fracture-associated lacerations of the lungs, lacerations or rupture of the heart, or laceration of the aorta. In a frontal impact, particularly for the driver, the heart and aorta can be crushed between the sternum and spine. Rupture of the heart often occurs in the anterior paraseptal wall of the right ventricle or in the thin-walled atria. Hemothorax may occur with rib fractures in the absence of obvious visceral injuries. One must not forget the role of lacerated intercostal arteries in the accumulation of blood in the chest cavities.
Laceration of the aorta tends to occur at the level of the ligamentum arteriosum, 2-3 cm distal to the origin of the left subclavian artery, as depicted in the image below. The mechanism is not completely understood, but it is generally felt that the anatomic fixation of the aorta at this point relative to its neighboring segments results in a differential in mobility during rapid deceleration, resulting in shearing and laceration. The laceration may be partial or only involve the intima. Sometimes, an externally normal-appearing aorta may have series of intimal stretch tears that are visible only when the lumen is opened.
Lacerations of the liver and spleen are common, especially of the dome of the liver and the junction of the right and left hepatic lobes. Contusions of the omentum, bowel, and mesentery are also seen. Higher energy impacts can lacerate the pancreas and kidneys as well. At high speeds, avulsion of the mediastinum from the spine, skeletonization of the pulmonary hilar structures, internal shearing lacerations of the lungs, and avulsion of the abdominal organs from their posterior retroperitoneal attachments can occur.
Fractures of the pelvis are also common. One mechanism is the impact of the knees against the dashboard, resulting in forces being transmitted to the pelvis. Acetabular fractures, pubic rami fractures, pubic symphysis separation (with frequent laceration of the urinary bladder), and disarticulation of the sacroiliac joints may occur. Removing the iliopsoas muscle group is required to adequately view the entire pelvic rim and sacroiliac joints.
Head and neck injuries
Facial and skull fractures are common. Skull fractures tend to occur in a plane parallel to the direction of force: frontal impacts can cause fractures in the sagittal plane; side impacts can cause fractures in the coronal plane (see the following image). Impacts of the chin can transmit force to the base of the skull and result in fractures through the base of the skull, the so-called hinge fracture.
Higher speed impacts can result in diffuse axonal injury (DAI), caused by shearing forces on axons and blood vessels. These injuries can be graded according to the level of severity and the distribution of the characteristic pinpoint and streak hemorrhages throughout the hemispheric white matter, the corpus callosum, and brainstem. Parenchymal tears with accompanying hemorrhage can also occur. 
Temporary ejection -- when the occupant exits the side window briefly in a rollover, and the rolling vehicle crushes the head -- will result in extensive crush injuries. In these cases, the decedent may remain in the vehicle when it comes to rest and have fatal crushing head trauma. A dent with blood and/or brain tissue may be found on the roof of the car over the
occupant's position. An example of a car in a rollover accident in which a temporary ejection occurred is shown below.
Whiplash injuries cause traction on the upper cervical spine and atlantooccipital junction. Atlantooccipital separation or injuries of the atlantooccipital apparatus can result in devastating injuries to the brainstem or upper cervical cord, resulting in rapid death (see the radiograph below). Complete separation is not necessary. Laxity in this region from ligamentous tearing can be enough to cause injury to the medulla oblongata. Occasionally, microscopic examination of the brainstem may be necessary to document hemorrhage. Transient mid and lower cervical spine separation can result in traction either on the spinal cord at that point or farther up, resulting in partial or complete separation of the pontomedullary junction.
Drivers can experience ankle fractures as their feet are caught under the seat. Impacting the knees on the dashboard not only transmits forces to the hips, but can drive the patella like a wedge into the femur, resulting in splitting and separation of the femoral condyles.
Mechanical or positional asphyxia can cause death, and a thorough scene investigation is paramount when making this determination, because the physical injuries may not be sufficient to explain death. An occupant can be forced into a small space due to intrusion of the passenger compartment, disallowing expansion of the thoracic cavity. (Scene investigation will be discussed in a separate article.)
In a rollover accident in which the vehicle comes to rest on its roof, the belted occupant becomes susceptible to asphyxia, particularly if he or she is obese or intoxicated.  If ejected from the vehicle, the vehicle can partially come to rest on the individual, again preventing expansion of the chest and abdomen. The author has seen one case of a muscular, fit, young man who was ejected and had his vehicle's roof come to rest on top of him with only his feet protruding. He had a few abrasions, but no visceral injuries or fractures. In these cases, petechiae are usually florid in the head and neck and can also be seen in the upper chest and shoulders (see the image below).
Ejection of occupants occurs with greater frequency in rollover accidents and side impacts that cause rotation of the vehicle, and ejection occurs with more than 7 times greater frequency when the occupant is not restrained (44% vs 6%). Furthermore, fatality rates are much lower for nonejected occupants as compared with ejected occupants in the same crash.  Temporary, partial, or complete ejection of an occupant usually results in extensive trauma. After being ejected, the individual can impact solid objects (such as walls, support columns, or trees), the vehicle may crush them during a rollover, or they may be impacted or run over by other vehicles.
As mentioned earlier, the driver may be found in the driver's seat after a rollover accident with extensive head trauma and a bloody dent in the roof over him. In this case, the driver was partially ejected and his head was caught between the vehicle's roof and the ground as it rolled.
If an occupant is ejected from the vehicle and slides along the roadway for a distance, there will be extensive, dense, confluent abrasions (brush abrasions). In these instances, despite the striking appearance of extensive cutaneous trauma, little to no hemorrhage may be seen in the subcutaneous tissues under these abrasions. Characteristic parallel abrasions caused by freeway rain grooves may also be present. The following 2 images demonstrate these abrasions.
Vehicle restraint devices: seat belts and air bags
The NHTSA estimates that in 2008, seat belts saved over 15,000 lives, and air bags saved over 2,500 (those older than age 13 years).  Although seat belts and air bags save lives, they can also result in injuries or death. However, estimates by the NHTSA show that air bag-related fatalities have been reduced significantly compared with those in the 1990s and have remained so for the last 9 or 10 years. 
Seat belt-related injuries consist of abrasions, contusions, or abraded contusions across the waistline and at an angle across the upper abdomen, chest, or neck, as shown in the following image. This pattern will also confirm or refute the occupant's position in the vehicle.
Internally, injuries including abdominal aortic laceration, iliac artery laceration, left atrial appendage rupture, ventricular rupture, intestinal or stomach injuries, and lacerations of the solid organs, particularly the liver can be seen.  Seromuscular tears -- injuries in which the submucosa and inner circular muscular layer of the intestines separate -- have also been associated with seat belt injuries. 
Improper use of the seat belt or failure of the seat back can result in "submarining," in which the restrained individual slides under the belt.
Air bags deploy at velocities between 100 and 200 miles per hour.  It is recommended that the driver and passenger sit as far back as possible (at least 10 in from the steering wheel) in conjunction with wearing a properly positioned 3-point seat belt to prevent contact with the air bag while it is deploying.  Although injuries and fatalities can occur secondary to air bags, their incidence has decreased dramatically since the early 1990s. Most fatalities are associated with improperly positioned or unused restraints or involve rear-facing child safety seats.  Children in the front seat are at particular risk, as, because of their shorter stature, the airbag will impact their head and neck instead of their chest and abdomen.
Reported injuries in the pediatric population primarily include cranial trauma, atlantooccipital dislocation, or upper cervical trauma. Adults can have similar trauma, but they have a higher incidence of thoracic injuries including aortic injures, pulmonic artery injuries, or flail chest. Abrasions on the upper neck and chin are also seen.  The chance of injuries is greater when the restraint systems are not used correctly. [5, 24]
The motorcyclist is the unrestrained rider of an inherently unstable 2-wheeled vehicle. All riders of motorcycles are ejected when involved in a crash. This creates an increased risk of injury and fatality similar to when motor vehicle occupants are ejected.
As mentioned earlier, the rider may wear full-body gear and a helmet, which can reduce or even essentially eliminate external trauma. Only an internal examination will reveal the extent of the injuries and the mechanism of death. Examination of the helmet will reveal impact sites and can help when attempting to interpret the underlying head trauma. Motorcyclists are especially at risk for head and neck trauma, with 75% of motorcycle deaths being due to head injuries. 
The interaction between the pedestrian and the motor vehicle and the resultant injuries are dependent on several factors, including the speed and type of the vehicle, adult versus child pedestrian (stature), and whether or not the driver was braking.
At city speeds, the pedestrian will follow a somewhat predictable pattern of impact with the vehicle. The bumper will impact the lower leg, and the hip or shoulder will impact the hood. The shoulder or head will then impact the windshield or A-pillar. The pedestrian will then be thrown to the side of the vehicle or forward. Two examples of typical motor vehicle-pedestrian impacts are shown below.
At freeway speeds, the pedestrian may be thrown up and over the vehicle and may impact the roof of the car before falling to the roadway. Subsequent vehicles may then impact or run over the individual.
When a larger, higher-front vehicle such as a semi truck tractor (tractor trailer) or van hits the pedestrian, the higher surface area will throw them forward. Bumper injuries of the legs may not be seen, and the injuries will have a more planar distribution along one aspect of the body, similar to a fall from height. At lower speeds, the pedestrian will be run over.
Children, having a lower center of gravity, will respond to a typical passenger car impact similar to the way an adult is struck by a larger vehicle.
Bumper fractures occur when the bumper of a vehicle impacts the lower extremity, usually the lower leg. A particular type of fracture can help determine which direction the pedestrian was facing relative to the vehicle at the point of impact. The tibia may sustain a wedge-shaped fracture, with the base on the side of impact and the apex pointing in the direction of travel (see the image below). [12, 13] Fractures are more common in the weight-bearing leg.
Higher speeds can result in shearing forces that separate the skin and subcutaneous tissues from the underlying muscle, creating avulsion pockets. Contusions, soft-tissue hemorrhage, and hemorrhagic avulsion pockets can also help in determining direction. The leg struck by the bumper may impact the other leg. In this situation, if the bumper came from the right, the lateral right leg will have a bumper impact injury, and the medial aspects of both legs will also have injuries.
When the pedestrian is struck from behind, superficial tears in the inguinal regions may be present. These are irregular, linear, superficial, parallel lacerations that may exhibit little to no hemorrhage, such as seen in the image below. Deep lacerations can be seen at higher impact speeds. If struck from the right rear side, more of these superficial lacerations will be seen on the left.
When a pedestrian is thrown up onto the hood and windshield, pelvic, spine, and rib fractures as well as visceral injuries can occur. The distribution of these injuries can also be useful in determining the dynamics of the impact.
Head and neck injuries are the most common cause of death in pedestrian accidents. Typical blunt force trauma such as scalp lacerations, facial and skull fractures, epidural and subdural hemorrhages, cortical contusions, atlantooccipital dislocation, and cervical fractures are common. Head injuries can also be sustained when the pedestrian falls onto the roadway after being struck by the vehicle.
Clues such as tire marks or grease on the skin or clothing are a sign that the person was run over, such as shown in the image below. As the tire moves over the head, shearing and avulsion of the scalp can occur. Children, due to the pliability of their skeleton, may have relatively few injuries externally, few if any fractures, and extensive visceral trauma when run over. Tire tread marks may be present.
Deaths due to fires represent 2-3% of all motor vehicle fatalities. And although collision-related motor vehicle fires represent 3% of all causes of motor vehicle fires, they account for the majority of the fatalities.  This is likely due to occupants becoming trapped in vehicles (due to intrusion or chassis deformation preventing the doors from opening) or they are incapacitated so that they cannot escape. An example of a vehicle that sustained a car fire is shown below.
Most fatalities in car fires result in full-body or nearly full-body charring, with relative sparing of the back if the individual is still in the car seat (see the following image). In these cases, identification becomes the most important issue facing the medical examiner's or coroner's office. After that, the cause of death must be determined. Was it because of the fire, blunt trauma, or both?
Similar to house fires, despite the external appearance of the charred body, the internal organs are generally in good condition, and blood and other body fluids are easily obtained for toxicologic testing. In addition to the usual testing performed in motor vehicle accidents, a carboxyhemoglobin concentration should be obtained in order to assess whether or not the person was alive in the fire. Soot in the airway is also helpful (see the following image).
Although carboxyhemoglobin testing is generally a reliable way of determining whether the person was alive in the fire, Hirsch et al described 6 simultaneous fire fatalities without elevated carboxyhemoglobin.  They speculated that (1) efficient burning of the fire in its initial stages resulting in little carbon monoxide production, (2) an outdoor fire with sufficient ventilation, or (3) "instantly lethal reflexes" initiated when the victim is exposed to a flash of heat may result in this finding. 
Conversely, the presence of carboxyhemoglobin does not necessarily mean the individual died because of the fire. Inhalation of carbon monoxide in the passenger compartment from a faulty exhaust system may cause death or incapacitation before the impact. Careful attention to the injuries and determining whether they are postmortem or antemortem are necessary to differentiate this scenario.
If carboxyhemoglobin is not detected, cyanide testing may be indicated, as certain materials in vehicles generate hydrogen cyanide gas. 
It may be challenging to determine if extremity fractures are due to blunt force trauma or related to the thermal contractures seen in charred victims. Hemorrhage around the fracture can assist in this determination.
Heat will shrink the dura, pulling it away from the inner table of the skull. This has 2 effects. First, congealed blood may accumulate in the epidural space. This blood is friable and different in consistency from the more liquid, "real" epidural hemorrhage. Second, the contracting dura will compress the brain and give it a flattened appearance. This is not to be mistaken for cerebral swelling. Heat can also cause fractures in the skull. These heat-related fractures typically have separations between the inner and outer tables of the skull.
Natural death while at the wheel
Sudden, unexpected natural death can happen at anytime, anywhere. Just as a person can collapse suddenly while walking down the street or while sitting at the kitchen table, he or she can have a sudden, incapacitating event while at the wheel of a vehicle.
In a 1968 study, West et al reported that over a 3-year period, 15% of California drivers dying within 15 minutes of their single-vehicle accident died from natural causes.  In a 15-year retrospective study, Buttner et al found that 0.4% of all cases at their German institute and 2% of all autopsies of deaths at the wheel showed the decedent died of natural causes 
Determining that the cause of death was due to natural causes can be accomplished by finding sufficient natural disease but insufficient trauma to explain the death. Historical factors such as suddenly becoming unresponsive or slumping over at the wheel or extremely low vehicle speeds before a soft impact can point the investigator in the direction of a natural death. However, a person can simply become unconscious due to cardiac disease and then be killed by the blunt impact injuries before succumbing to his or her myocardial infarct. Asphyxia as a mechanism must be excluded before certifying a natural death.
In pedestrian fatalities, one must dissect the lower legs. A posterior incision can be made and the skin and subcutaneous fat should be dissected off the musculature. The location of any avulsion pockets or crushed tissue should be noted, as well as their height above the bottom of the heel. The height of fractures above the heel should be recorded; however, fractures may not occur at the point of impact. In the case of wedge-shaped bumper fractures, these do occur at the point of impact and can also provide information about the direction from which the impact came. [12, 13]
Dissection of the posterior neck should be performed in pedestrians who do not have otherwise obvious causes of death. Hemorrhage may not be visible anteriorly. Stretching and tearing of the ligaments of the atlantooccipital junction and C1-C2 may occur without complete separation. Even laxity in this region can create traction on the brainstem and upper spinal cord. The spinal cord can be exposed by cutting through the laminae posteriorly. Cutting a triangle at the base of the occipital skull can create more exposure and ease direct visualization of the odontoid process.
Removal of the iliopsoas muscle group allows complete visualization of the pelvic rim and sacroiliac joints; fractures of the pelvis can be missed without taking this step.
Special autopsy procedures
It is vital to examine the clothing in pedestrian deaths. Tire marks, trace evidence, and other changes on the clothing can help in the investigation of a hit-and-run accident. Suicide notes will be missed if the clothing is not searched.
Histology and Microscopic Examination and Findings
Microscopic examination of the brainstem, specifically the medulla oblongata can reveal microscopic hemorrhage that is not clearly evident grossly.
In fatalities that are delayed by hours or a few days, fat emboli may be demonstrable in the pulmonary or cerebral vasculature. In the brain, they may be seen as pinpoint hemorrhages in the deep white matter of the cerebral convexities.
Evidence of diffuse axonal injury can be seen microscopically with routine hematoxylin and eosin (H&E) stain if the individual lives 12-24 hours or more after the injury, manifested by eosinophilic axonal spherules in the white manner of the brain. Typical locations include the centrum semiovale, the corpus callosum, and the brainstem. Before 24 hours, beta-amyloid precursor protein (beta-APP) can be used to visualize the spherules, but they do not become visible with beta-APP before about 2-4 hours post injury. [15, 30]
Photography and Documentation
Photographic documentation of the body is paramount. Even the longest and most descriptive report will not convey the information a single photograph can. Pay attention to the body before undressing and washing. Grease stains and tire marks may only be visible on the clothing and may wash off. Overall, photographs are best for recording the distribution of the injuries on the body. Close-up images can document patterned injuries.
One must always generate written documentation with diagrams and measurements. Despite the general reliability of data storage, especially when on a proper server, data fail. In other words, as digital photography becomes more pervasive in the forensic world, accurate and detailed paper diagrams still remain important. Recreation of a lost report or dictation file or tape can be done only with proper documentation.
Recording measurements in inches can be helpful for juries, lawyers, judges, and law enforcement officials who might not be as familiar with the metric system.
Ancillary and Adjunctive Studies
Obtaining appropriate specimens for toxicologic testing is a critical component of the role of the coroner's or medical examiner's office. Most commonly, interest is centered around the blood alcohol concentration. However, testing for common illicit drugs or medications that may alter mental status or affect the driver's ability to operate a vehicle (eg, opiates, benzodiazepines) is also important to consider. In the case of natural disease, vitreous testing for glucose (coupled with ketones in the blood) can reveal evidence of diabetic ketoacidosis.
Manner of death
Motor vehicle fatalities are generally, by convention, certified as accident, despite the definition of homicide as "death at the hand of another." When a police investigation reveals that the car was intentionally used as a weapon, a certification of homicide is warranted. Motor vehicle-related fatalities during the commission of a crime may also be certified as homicide.
In single-vehicle crashes, one should consider the possibility that the driver intentionally took his or her own life. A history of depression or suicidal ideations can help in this determination. Lack of skid marks before impact (see the following image) or identifying the imprint of the gas pedal on the bottom of the shoe also points one in the direction of suicide. Suicide notes may be found in the clothing of the decedent.
Issues Arising in Court
Many facets of an investigation may become important in legal proceedings. The surviving, intoxicated driver of a vehicle in a crash that results in the death of a passenger may say that the decedent was the driver. Close attention to the distribution and type of injuries (eg, dicing, steering wheel injuries) in vehicle occupants can provide important clues regarding their position in the vehicle.
In pedestrian fatalities, the direction from which the individual was struck will come into question. The height above the heel (plus the height of the shoe heel) can be important in determining if the brake was being applied at the time of the impact, implying that the driver did or did not see the pedestrian.
The cause of the accident and the cause of the death are not synonymous. The cause of the accident can be hazardous roadways, driver error, mechanical failure, impact from another vehicle, or a sudden incapacitating natural event. The cause of the death is the disease or injury -- physical or chemical -- that initiated the lethal sequence of events. Court inquiries often center on the cause of the accident, but the cause of death can be integral to this determination.
Natural disease and toxicologic issues can provide critical clues when trying to ascertain the factors that contributed to the cause of the accident. They may also help explain how the death occurred in a situation in which the injuries do not at first glance appear fatal.
When how much "pain and suffering" the decedent sustained before their death is an issue in court, the one who understands the injuries and the complex interaction with any natural disease or toxicologic findings may be best positioned to answer this question, especially when the injury would have resulted in uniform, rapid death.