Sometimes after a collision has occurred, there is an uncertainty around who was actually driving at the time of the crash. We call this, as you probably do too, the “Who was driving?” scenario.
The “Who was driving?” scenario arises when:
1. No one admits to being the driver;
2. Everyone is pointing their finger at someone else;
3. All survivors are blaming someone who was fatally injured in the crash;
4. The driver left the scene after a crash, such as hit and run or a crashed and abandoned car;
5. Something is suspicious or just isn’t adding up about who claims to have been driving.
Cases where the driver is in question come up most often when one or more of the following features apply: alcohol is a factor, the owner reports that the car had been stolen, and/or there has been a single vehicle collision into tree, pole, or ditch. The big question is, can forensic experts tell who was sitting in the driver’s seat when the crash happened? The answer is … sometimes! Scientifically determining occupants’ seating positions at the time of the crash is akin to solving a mystery. If we have enough of the right clues (evidence), we can do it, based on science!
First, there’s more than one impact to be considered
When a car crash takes place, there are two different but related “impacts” – the vehicle crashing into another vehicle (or object), and the occupants colliding with the interior of the vehicle. (Technically, there is even a third impact to be considered, wherein the internal organs of the occupant move and can be stretched beyond their limit or collide with other organs or the musculoskeletal system. But for the sake of simplicity, we’ll be focusing on the first two “impacts” mentioned).
In most cases, it may very well be necessary to first preserve and analyze the scene and vehicle evidence in order to reconstruct the crash in terms of its vehicle dynamics (movements, speed and trajectory, etc). Vehicle dynamics gives us critical information about what happened to the vehicle first, which forms a foundation for the understanding of what happened to the occupants inside. This is all thanks to Newton’s first law: an object in motion remains in motion until acted upon by an external force. A vehicle and an occupant inside travelling down the road together and are at equilibrium with each other; there are no significant accelerations and/or forces between them (other than normal driving vibrations and manoeuvres). If the vehicle is braked using emergency braking or is in a frontal crash, the occupant inside will continue to move along the original direction of movement until an external force changes that – an external force, which we always hope is a seatbelt rather than a steering wheel, dashboard, or windshield. Typically, the more complex the vehicle dynamics, the more complex the occupant movements are. For example, the movement of an unbelted driver in a rollover is much more complex than a belted driver in a frontal impact, although Newton’s first law applies to both equally.
Whatever the complexity, we usually need to start by putting on our forensic physics cap to look at that first “impact” (the vehicle crashing into another vehicle or object, or into the ground during a rollover). We take a detailed look at all the available evidence altogether here: the vehicle damage, exterior and interior, EDR “black box” data, and the environment and scene evidence. We reconstruct the vehicle collision and in as much detail as necessary. We calculate the impact speeds and severities, as well as the directions of forces involved. We simulate the vehicle movements and rotations on three-dimensional terrain.
Having deduced the vehicle dynamics portion of the collision, we switch and put on our forensic biomechanics cap to study the second “impact” – that of any occupants with the interior of the vehicle. This may include a more complex forensic biomechanical simulation of the individual body segments, as well as the application of research on impact tolerances and injury probability from human subjects and crash test dummies.
Generally speaking, occupants will travel towards the point of impact (see figure 1 below) until they are stopped by the interior of the vehicle – which could be a seatbelt, dashboard, steering wheel, seatback, airbag or even another occupant. This is where the medical information about the occupants involved in the crash is reviewed and compared to any physical evidence of contact (or lack thereof) in the interior of vehicle.
Figure 1: Occupant Kinematics 101
Solving the Mystery
Finally, we can zoom in and take a look at the occupants’ specific injuries in the context of how we know they would have moved in the vehicle (based on the dynamics). For example, when a vehicle hits a tree, we expect a certain damage pattern to occur; likewise, when an occupant hits a steering wheel or a seatbelt, we expect a certain injury pattern to occur.
Let’s consider a simple hypothetical scenario: if someone claims that they were in the right front passenger seat and wearing their seatbelt during a severe frontal collision, an injury to their right shoulder/collar bone area would be expected. If, however, their medical records show an injury to the left collar bone, that wouldn’t match their claim of being belted in the front right seat. Rather, a left collar bone injury is something we would expect to see if they were belted in the driver’s seat or the left rear passenger seat (where the seat belt would have crossed their torso from the left shoulder). So, the medical / biomechanical evidence is indicating that they were actually sitting in the driver’s seat or the back left seat – but which one was it? If the vehicle was newer with abundant EDR or “black box” data – there is a good chance that the black box could bridge the evidence gap and provide data on whether there was a belted occupant in the driver seat (and sometimes their size classification). This chain of evidence can be linked together, along with other evidence, to form a solid argument that they were the driver.
Seatbelt injuries can be a pretty big “giveaway,” but they aren’t the only ones associated with certain seating positions. Many injury types and locations can give us hints as to where that person was seated and whether they were wearing their seat belt. Just as there are injuries we’d expect for a belted person, there are injuries we’d expect for an unbelted person.
Another piece of the puzzle to consider is that an unbelted occupant becomes a projectile in the vehicle during a crash. An unbelted occupant can injure others, which should always be considered if it seems like an injury doesn’t “match” a seating position. Here’s an example to illustrate this: Following a frontal collision, the passenger in the right front seat has a large bruise/swelling on the back left side of their head (a type of injury that results from direct contact). From a forensic biomechanics perspective, there are no movements or interior components that would have caused a direct contact type of injury in that location. However, knowledge that there was an unbelted back-seat passenger can explain the right front passenger’s injury. As the unbelted back seat occupant was thrown forward, they likely struck the right front passenger in the head, causing what would otherwise be an unexpected injury.
Since we’re on the topic of unbelted occupants, it’s worth mentioning that even if they were partially or fully ejected from the vehicle, where they were originally seated will often be signalled by vehicle damage, injuries, and landing location relative to the vehicle’s final rest position.
Lastly, organic materials such as blood, tissue and/or hair, on airbag fabric, seatbelt webbing, seat and other fabrics or components from the vehicle can be preserved as evidence for microscopic and/or DNA testing. If there are any viable samples on these components, they can be compared to DNA swabs of the occupants to match to the sample on the vehicle.
When a “who was driving?” situation arises, experts can analyze the “impacts” first from a crash reconstruction perspective, then by applying forensic biomechanics and looking at the occupants’ injuries in the context of the vehicle dynamics of the crash. Like a good detective, they can solve the mystery of who was driving when they have enough clues and evidence – and the more clues there are, the better the scientific certainty.