Three Collisions in a Crash

There are actually three collisions in every crash and as a paramedic it is vital to keep all three in the back of your mind when you’re assessing the mechanism of injury of a motor vehicle accident and searching for injuries in a patient. The three collisions include: the vehicle collision, the human collision and the internal (organ) collision.

In order to understand the three collisions in a crash it is important to have a basic understanding of kinetic energy. Kinetic energy can be defined as the energy built up in a moving object. When an object collides with another object the kinetic energy must be dispersed or the original object will continue to move in the original direction. The amount of kinetic energy required to be dispersed will depend on the mass of the objects in motion and the velocity (speed) in which the object is travelling.

For example, a car that weighs approximately 1200 kgs that is travelling at a speed of 50 kms/hour will have much less kinetic energy than a truck that weighs 40, 000 kgs and is travelling at the same speed of 50 kms/hour. Consequently, the car will require less kinetic energy in opposition to decelerate and stop than the truck.

When a vehicle is travelling down a road it has a specific amount of kinetic energy (based on it’s mass and velocity). If the vehicle crashes, this amount of kinetic energy will have to be dispersed in order cause the vehicle to come to a stop. In a motor vehicle accident, this kinetic energy is generally dispersed through the vehicle/object crushing, sound waves, and the development of heat. 

The Three Types of Collisions in a Crash

In an example of a motor vehicle crashing into a solid concrete barrier these are the three types of collisions seen. As a paramedic, it is important to understand these three collisions.

The Vehicle Collision

The vehicle collision is where the vehicle collides with another object (the concrete barrier) and the vehicle frame is crushed. Fortunately, most modern cars are designed to crush well and therefore absorb the majority of the kinetic energy, allowing the bulk of it to disperse before reaching the much more fragile human occupants.

The Human Collision

As the bulk of the kinetic energy is used up during the vehicle crash and the vehicle itself starts to decelerate to a stop, the second collision occurs, and this one involves the movement of the human occupants within the inside of the vehicle. While the crushing of the vehicle’s exterior absorbs the kinetic energy, the human occupants continue to travel in the same direction at a relative velocity, until they hit an object in the vehicle that causes their forward motion to stop. For most human occupants this will likely be due to the human collision with the stationary seatbelts. However, for those occupants who are not wearing a seatbelt, the next collision is likely to be the steering wheel, dashboard, or windsheild. The unrestrained occupant will likely be ejected from the vehicle or collide with the steering wheel or dashboard. When a human body collides with a rigid object (such as the steering wheel, dashboard, concrete barrier, or tree the motion of person travelling forward will cease almost instantaneously. This will result in the sudden disbursement of the kinetic energy, and due to the softness of the human body, will most likely result in the body crushing instead. This is why unrestrained occupants have the highest risk of death during a motor vehicle accident. It is also important to recognise that, although vehicle seatbelts appear rigid they have a certain amount of stretch that allows the kinetic energy to disperse over a greater duration of time.  

The Internal Collision

Once the occupant’s body has stopped movement the internal organs of the body still remain in motion in the original forward direction until another organ or body part exerts enough energy to cause it to stop. In this case a sudden stop of a motor vehicle into a concrete barrier would cause the vehicle to crush, followed by the seatbelts to restrain the human body, where upon the organs within that body would have to potential to tear, rupture or collide with each other. This is why motor vehicle accidents that involve high deceleration forces often lead to a ruptured spleen, liver, torn large vessels (such as the aorta) and brain injuries as the soft brain continues forward while the solid skull is forced to stop suddenly.

At the end of the day kinetic energy doesn’t disappear, it has to be acted upon by an external force in order to cause an object in motion to come to a stop. As a paramedic, it is vital to have a thorough understanding of the three collisions in a crash in order to comprehend how this kinetic energy is most likely to have been dispersed. Based on this knowledge, a good paramedic will be in a better position to predict likely injuries based upon the mechanism of injury and therefore determine a better treatment and transport decision for his or her patients at a motor vehicle accident.