The Three Collision Rule

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.

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  1. Great discussion! One I think is overlooked sometimes. Also I would like to add ina rollover scenario the 3 collision rule is in effect for each time the car rolls. I dont mean complete revolutions but each time a side impacts the ground. So it is possible in a full rollover (vehicle coming to rest back on its wheels) that 4 collisions happened thus rule of 3 would make 12 total collisions!!! Sometimes we cant see the trees through the forest.

    It is amazing to me that sometimes at MVAs folks go rushing headlong into gettting to the patient and doing “their thing” but dont really LOOK at what happened. Yes a glance around for scene safety of course but not, as you stated, skid marks speed limit signs ect. With just a minute of pause you can anticipate what your patient will be like before you approach the vehicle. What immediate threats will likely be need to be attended to. If I am at a scene where I have a few hands with me I will let them go ahead while I stand back and actually take notes. Not only does the ER Doc thank me but helps me and my crew help the patients better by knowing where and what even though we may be enroute already. Also try to get notes BEFORE rescue crews start tearing into things. Nothing like trying to figure out who was restrained after rescue cut every belt OR how bad the intrusion was when they just cut half the car back.

    I actually know one crew that caries a digital camera to snap pics. Let me stop right here and add this… immediatly after the call the memory card is erased by the chief. Only time it is left (photo that is) is if it will be pertanant to the ER Doc. Then it will be erased. Always witnessed by a third party and documented. HIPPA you know. Also never a picture of the patient, these are scene pics froma distance, just a reference basically, unless it is pertant then usually its attempted to keep the patient out of the photo.

    Ok back on track….

    Thanks for keeping up the Basics! I know alot of times folks move to the ALS side of calls and while all well and good without the basics stuff wouldnt be found. (no this is not a shot at als vs bls) Everyone needs to understand the mechanics to be able to treat.

  2. Yeah, My instructor taught us this during my basic class. The only thing I’d mention is to be sure to check for skid marks as that can give an idea if the driver(s) saw it coming and tried to slow down, and the direction initial impacts occurred

    I had a MVA where there were 3 cars involved. The first car ran a red hit the car that had the right of way and then hit a car who tried to swerve out of the way of where they thought the collision was going. The swerve patterns gave us a very good idea of how many collisions each car had even when the body damage didn’t.

    @Jude: Our department rigs each have a digital camera and the hospital loves us for it especially in circumstances with delayed extractions and multiple cars involved. Our department has also been cleared to save the pictures for training so long as we strongly blur the license plates, street signs and people’s faces.

  3. Very good article indeed. This three collision rule was stressed upon us in school in 2010 so it’s good to see that these kinds of things stay around in the EMS system for many years and continue to be taught today. Now I only say this from a student perspective (1 month of practicum to go) and I am not disregarding this article at all, but one more thing to keep in mind would be not to get tunnel vision by trying to predict in your head what kind of injuries your patient/s will have. A FULL rapid trauma survey is still warranted and should not be swept aside for a focused exam because of predictable injuries from a thorough scene survey. I know this was not your point at all I just wanted to mention it so we all will not get tunnel vision as it so often happens in EMS. Thanks.

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