Time and distance are key factors in understanding how the severity of collisions and injuries differ from one incident to the next.

Internet comments over the years demonstrate that the very large number of persons communicating between each other do not understand the basics of how to interpret clues about motor vehicle collision severity and the likelihood that serious injuries will occur. For this reason this short article will provide a very basic commentary about two key factors, time and distance, that influence the severity of a collision and whether serious injuries could be expected. Some simplifications are necessary to reduce the length of this discussion.

Why humans get injured in motor vehicle collisions is because our bodies are in motion, at the same speed as the vehicle we occupy. Because of this motion we possess kinetic energy, just like the vehicle that we occupy. Collisions cause our motion to be reduced, often in a very short time and distance. Thus our kinetic energy also must be lost in a very short time and distance. But these short times and distances mean that the acceleration imposed on our body must be very high and our bodies cannot withstand these accelerations without causing injury.

Thus a primary observation in assessing the severity of a collision, and the potential for serious injury, is to examine the time and distance within which a collision has occurred. To shorten this discussion we assume that we are looking at a head-on collision such as the one pictured the photo above. The key observation to make is that increasing the time and distance in a collision reduces the collision severity and also reduces the potential for occupant injury.

We often look at a photo such as the one above and come to the conclusion that, if there appears to be a lot of damage then this must have been a severe collision and one which should cause serious injury. In many cases this is true. But the time and distance within which the collision took place must also be considered. The severity of the above collision can be different even though the damage is the same. Why? Because the time and distance within which the collision occurred could be different depending on the vehicle or object that the vehicle struck.

If the vehicle shown above had struck an immovable, concrete wall then the vehicle’s speed would be lost in a very short time and distance and the collision severity and injury severity would be high. Conversely if this vehicle had struck a very soft object such as a yielding roadside barrier or a line of small trees, the damage would occur over a longer time and distance and the severity of the collision would be less.

In many serious head-on collisions the elapsed time between initial contact and separation (i.e. the time of the collision) is just over 100 milli-seconds, or just over 1/10 of a second. That is a very short time. Conversely, some head-on collisions involving soft structures or incomplete overlap can increase that time of collision to as much as 300 milli-seconds. That difference between 100 and 300 milli-seconds appears to be small but in fact it is huge. Often a vehicle occupant begins to move forward within the vehicle interior at about 60 milli-seconds after initial contact. And air-bags deploy in about 50 milli-seconds. All these numbers sound small they are hugely influential in determining the level of occupant injury.

The severity of the collision experienced by the vehicle that we occupy is not the same as the severity of collision that our bodies experience. The damage we see on a vehicle only demonstrates the severity of the collision experienced by our vehicle. But the severity of the collision experienced by an occupant differs depending on factors such as how quickly we begin our deceleration with respect to the initial contact. If we use our seat-belts, and they are properly positioned tightly to our body, then we become a part of the structure of the vehicle in the sense that our bodies begin to decelerate after only a short delay after initial contact. If we do not wear our seat-belts we do not begin our deceleration until we begin our motion within the vehicle interior and then strike some portion of that interior. This longer delay is very important because, by the time our body reaches contact with the vehicle interior the vehicle has already lost a large portion of its speed. This means that the difference in speed between the vehicle interior and our body is much larger and we sustain greater injury.

So let us leave this brief discussion at these few concepts. There are many other matters that would need to be discussed before an reasonable understanding of collision severity and injury severity can be gained. But we can review these basic concepts. The severity of a motor vehicle collision experienced by a vehicle is not the same as the severity experienced by a vehicle occupant. But both are influenced by how much time elapses and how much distance is travelled during the time of contact.