Speed Can Be Irrelevant In Injury Causation

While the title of this article sounds completely wrong it is an important lesson to understand why it is mostly right.

It is never convenient to discuss the results of real-life motor vehicle tragedies because they always involve real people. Yet a failure to educate ourselves from these results often leads to future tragedies that could otherwise be prevented.

The lesson in the present article is an attempt to clarify a common misunderstanding that the travel speed of a vehicle is what causes injury. In other words, if you are travelling faster you will sustain a higher severity of injury. This fallacy has been growing deeper roots in some recent research and has been picked up by news media resulting in unreasonable demands for policy change that hinders true improvements in road safety.

The above photo shows a recent tragic scenario where an SUV carrying 3 occupants crashed into a tree on Webb Road in Durham, Ontario resulting in one fatality. The photo of the vehicle at its final rest position shows that its driver’s door was pressed against the large tree and there was massive crush into the driver’s seating area. Most persons looking at the photo would conclude that the SUV must have been travelling at a tremendous speed in order to cause the resulting damage. In reality these views of the vehicle cannot be used to draw that conclusion.

Let us examine a typical scenario where an SUV is travelling on a typical Ontario highway with a posted maximum speed of 80 km/h. Let us say that we know its speed was 100 km/h. As it passes through a  curve the driver loses directional control of the vehicle and it begins to rotate while the driver steers left and right to regain control. This action may take place over a distance of 100 metres. While this “yawing” motion is taking place the vehicle is losing speed, often at a rate between 0.2 and 0.4 g. Taking an average of 0.3 g in that distance of 100 metres the vehicle would lose a speed of about 87 km/h. The vehicle then strikes a tree with its driver’s door and the vehicle comes to rest against the tree. The relatively low speed of impact results in relatively minor damage but because the tree is pressed against the driver’s window, and because the driver’s body moves toward the point of the application of the force, the driver’s head strikes the tree and the driver dies. Looking at the minor damage to the vehicle many would scratch their heads and say “there must be something wrong here. A driver should not die from such a low speed impact”.

We then look at a second example, the same SUV is driving the same road at 100 km/h in wintertime and the same loss-of-control takes place due to an icy road surface. Because of the ice the vehicle might slow down at a rate of only 0.05g over the 100-metre distance. Calculating the speed loss we obtain about 36 km/h and the vehicle is still travelling at about 64 km/h when it strikes the tree. But due to the luck of chance the impact occurs just behind the driver’s door and the massive intrusion to the structure is located behind where the driver is seated. The driver survives and everyone wonders: “Wow, how could this person survive such a high speed impact?”.

What can we learn from these results?

In both instances the SUV was travelling at the same speed. But in the first instance the vehicle lost a substantial amount of that speed due to its sliding on a surface that was able to dissipate (remove) its kinetic energy. Thus the initial speed of the vehicle was not relevant. What was relevant was the speed when it struck the tree. Not only was the impact speed an issue, but also the location of the application of the force: directly at where the driver was seated.

In the second impact the SUV’s speed was not reduced appreciably before the tree impact and the impact speed was much higher. But because the impact was behind the seated position of the driver the crush or structural intrusion was not applied into where the driver was seated. The collision appears to be very severe but the driver survives.

This illustrates the fallacy of using initial speed of a vehicle in discussing the danger of sustaining significant injury. The initial speed in not the important factor, it is the impact speed that is important. Furthermore it is the change-in-speed that is important. As we dig deeper we then begin to add the fact that it is how quickly that change-in-speed occurs that is the critical factor. And we then add that it is also the direction of force, the existence of pre-impact rotation, and so on, that need to be taken into consideration.

Now, before ending this discussion we need to clarify some additional points. When we talk about injury causation we do not discuss change-in-speed. Instead we use the term “change-in-velocity”. Speed is a scalar term that does not give an indication of the direction of that speed. Velocity is a vector and is a more complete term because it has the associated direction applied to its definition. So a speed may be “100 km/h”, while a velocity would be “100 km/h in a north-west direction”. The directional component is an important matter that cannot be ignored.

We then need to add that it is not  the change-in-velocity that is relevant but how quickly that change-in-velocity occurs. So this is where we introduce acceleration. Acceleration is the rate-of-change in velocity.

And further still be might add that the change-in-velocity or acceleration experienced by the body of the vehicle is not the same as the change-in-velocity or acceleration experienced by the occupant inside the vehicle.

These are just some of the basic and simple issues that need to be understood when discussing injury potential and the travel speed of a vehicle. We need to understand that we cannot equate travel speed with injury causation. There is no direct relationship between these terms. The title of this article is somewhat misleading because, indeed, if you are travelling 100 km/h you possess the potential ingredient for causing serious injury and death. But it is not true that you will sustain serious injury or fatal injuries just because of that travel speed. The issue is more complicated than that.

Another issue that has been prominent in recent fatal collisions is the issue of initial speed, structural separation and injury causation. This point is highlighted by two attention-grabbing collisions that occurred, one Edmonton Alberta, and one in Toronto Ontario the past couple of months.

In an article that was posted to this website on July 5, 2020 we attempted to explain that three occupants of an Audi were killed after impacting a Starbucks retail outlet, not because of the vehicle’s speed, but because of the disintegration of the vehicle’s structure. Again, this was meant to highlight the important issue that we need to look at the severity of the impact in terms of the change-in-velocity experienced by the vehicle. When there is a lack of evidence showing that a large amount of kinetic energy was dissipated we need to ask more questions about why a fatality occurs.

This concern was repeated once again in another Audi impact that occurred in Toronto, Ontario. The driver of an Audi lost directional control of the vehicle and it rotated into the path of a Jeep SUV. It appeared to all involved that the speed of the Audi, as its driver lost directional control of the vehicle, was all that mattered. The fact that the Audi separated into two pieces seemed acceptable because the Audi was travelling at a high speed.

While the last views of the Audi appear to show an unsurvivable amount of destruction, the driver reportedly survived as well as one of the three occupants. So how severe was this impact? Vehicle occupants die because they are involved in collisions where a vehicle sustains a high change-in-velocity. Did this Audi sustain a high change-in-velocity. If so, why did two of the three occupants survive?

Many vehicles contain weak points in their structure. When a vehicle is struck in the side, in the manner suggested in these photos, structural separations occur. Not just today, or yesterday, or last year, or 10 years ago. Vehicles have been separating like this since they began to be manufactured as unibodies.

We do not have the privilege of examining the details of this crash. Only the investigating police have that opportunity. This leaves us at a disadvantage as we can only use the photos of the damage and the vehicle rest positions to support our comments. However collisions of this type often happen in a recurring manner.  When a vehicle rotates out-of-control, it travels into the opposing lane while continuing that rotation. The manner of contact to the vehicle’s side is not the same as the scenario encountered in an intersection collision. Nor is it in the manner in which vehicle crashworthiness is tested in government crash tests. It is likely that the pre-impact rotation of the loss-of-control vehicle is part of that difference. It has also been observed that the direction of the impact force contains a component that involves “sliding” or sideswiping across the struck surface.  The combination of these factors leads to a loading of the side structure in a manner that causes the separations observed in the Edmonton and Toronto collisions. This weakness in the vehicle structures causes massive destruction that, to the average onlooker, appears to be consistent with a high speed of a vehicle and the crashworthiness of the structure is never questioned.

To summarize, it is an attention-grabbing comment to suggest that speed is not relevant to injury causation. In one way the comment is in error because it is the speed which provides the potential for injury causation. But we must be careful to understand that speed alone is not the critical factor. It is what happens to a vehicle’s motion during an impact that matters in injury causation. And it is what happens to the body of the vehicle occupant which makes contact with the vehicle interior that matters in injury causation. These issues may be completely irrelevant to the ground speed of the vehicle.