I imagine it was probably my first Prehospital Trauma Life Support class back in 1990 that I first heard of the three collision rule. Since then, it has remained a useful tool in examining the mechanism of injury after auto accidents. If you haven’t heard of it, please allow me to elaborate.
The three collision rule states that, in any auto accident, there are three collisions that occur and the keen EMT needs to be aware of all three. The next time you walk up to a vehicle accident, instead of imagining two large objects colliding with each other, imagine three separate collisions occurring with each respective vehicle. All of them have implications for the alert EMT.
Collision Number One: The Exterior of The Vehicle Strikes Something
Take a quick walk around the vehicle and consider the elements of Newton’s second law (force is mass times acceleration or deceleration). Ask yourself the questions that apply to that equation. How heavy is the vehicle involved? How fast was it traveling? How fast did it stop? All of these will contribute to the force involved in the initial impact, but speed really is king. Force increases proportionally as the weight of the vehicle increases, but speed has an exponential influence on collision forces.
Several things can give us clues about how fast the car was traveling at impact.
First, pay attention to the posted speed limit in the area of the collision. Cars traveling through a 25 mile per hour residential neighborhood will collided with one fourth the kinetic energy as a car driving through a fifty mile per hour commercial district. Second, look for skid marks and other indications of attempts to slow the vehicle before impact. Third, look at the vehicle itself. Keep in mind the older cars are designed to resist impact forces and newer cars are designed to absorb impact forces while keeping the passenger space intact.
In modern cars, passenger space intrusion and axle displacement are significant findings. Also note airbag deployment within the passenger space and if the headrest height is in proportion to the driver. Now we can move inside the vehicle.
Collision Number Two: Stuff Within The Vehicle Moves Toward The Point of Impact
This includes the vehicle occupants. Everyone inside the vehicle will move in the direction of impact until they are stopped by their seat belts or the inside surface of the vehicle. Both of these options can create injuries. Keeping that in mind, it’s useful to know where the patient was sitting inside the vehicle and whether or not they were restrained.
Also peak around for other heavy objects that may have struck the patient during the crash. Groceries, car seats, tool boxes and the bowling ball from last weeks league game can all become deadly projectiles during a run-of-the-mill vehicle accident. In much the same way, oxygen bottles, med kits and ECG monitors can all add complications to a run-of-the-mill ambulance accident. I digress.
Collision Number Three: Stuff Within The Patient’s Body Strikes The Inside of The Patient’s Body
Excuse my overly technical use of the word stuff, but there’s a lot of stuff inside the average patients body and it all behaves differently when it’s forced to go from fast-forward to stopped-dead. Solid organs like the spleen and the liver fracture and bleed. Hollow organs like the stomach rupture. Vessels like the aorta tear. Lungs rupture or become punctured.
This is all part of the third collision. And if you’ve paid attention to the first two collisions, much of it is predictable. Certainly you can’t predict all the injuries suffered in an auto accident, but you can get pretty good at guessing what injuries you might find based on a thorough investigation of the mechanism of injury.
There you have it. The three collision rule. The next time you walk up to a car accident, count the number of cars and multiply by three.
What do you think? Do you know the three collision rule? Do you think about it? Is it useful? Leave a comment and join the discussion.