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So I am sitting at traffic lights, stationary, in my car.

A drunk driver has crossed over and is about to hit me head on at 40 mph.

I only have enough time to do one thing, hit the brakes or leave them off. Which should I do to minimise injury to myself? Are they the same?

The case for hitting the brakes is that I am stationary and by hitting the brakes, I will remain as close as possible to stationary as I can be. Thus less has happened to me?

The case for leaving the car free to roll is that, in an ideal world, I would reverse to 40 mph and simply never be hit. Braking is to apply a force in the opposite direction (i.e. forward) while reversing would be to apply a force in the other direction. So leaving the car free to roll is as close to reversing as possible. Thus as close to optimal as possible.

My guess is that it doesn't matter, the maximum force (acceleration) I will feel is going to be the same but perhaps someone has come across this before and can shed more light.

Any ideas appreciated.

Leonhard Euler
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  • Geometry of the impact is probably more important. Cars are designed to crumple in some directions and not in others. So a head on is going to have two crumple zones, but being t-boned you only have one. – zeta-band Nov 27 '18 at 18:12
  • @zeta-band In both scenarios, it is a head on collision. – Leonhard Euler Nov 27 '18 at 18:34
  • I don't know if this is really on topic here. This situation would depend significantly on the crumple mechanics of the vehicles, and is complex enough that you could likely to multiple experiments of this with near very similar setups and get different results. To me, it seems like something you would need to examine on a model by model basis, or at least have experimental evidence to back up an answer. I get the feeling that car manufacturers would know the answers for their vehicles in a number of situations. – JMac Nov 27 '18 at 19:49
  • I find it a nice question. Think of two spherical cars, and model the driver as a pendulum inside one of them. – scrx2 Nov 27 '18 at 19:57
  • @scrx2 This question isn't asking about the movement of pendulums hung in spherical cars. If it was, I wouldn't have made that comment. – JMac Nov 27 '18 at 20:31

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The case for hitting the brakes is that I am stationary and by hitting the brakes, I will remain as close as possible to stationary as I can be.

This is what you want. The least possible energy transferred into your body.

The case for leaving the car free to roll is that, in an ideal world, I would reverse to 40 mph and simply never be hit. Braking is to apply a force in the opposite direction (i.e. forward) while reversing would be to apply a force in the other direction. So leaving the car free to roll is as close to reversing as possible. Thus as close to optimal as possible.

This is the way to minimize the damage to your car, since it allows some energy to be transferred into moving the car rather than deforming the car. But it's not the best way to minimize damage to the occupant(s).

If you were to put your car into reverse and get out of the way, the acceleration your body experienced would be limited by the ability of the engine to accelerate the car.

If you leave the brakes off and let the other car hit you, you will be accelerated much more rapidly, which is likely to lead to injury (because not all parts of your body will be accelerated equally).

The Photon
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    I am puzzled by your second comment. The sitting driver is part of the car. If part of the energy goes into kinetic energy of the sitting duck car, it is not available for deformation, including "deformation" of the sitting driver.. I think rolling is less damaging both to car and driver. – anna v Nov 27 '18 at 18:45
  • @annav, Best case scenario (or limiting case), if you put on the brakes, all the energy goes into deforming the car, and your body is neither accelerated nor deformed. – The Photon Nov 27 '18 at 18:59
  • If you allow the car to move backwards, the total energy dissipated will be less because the overall change in kinetic energy will be less. This doesn't seem to show enough justification that the lessening of impact due to slightly restricted movement could outweigh the energy that must be dissipated. – JMac Nov 27 '18 at 19:11
  • @JMac, it doesn't matter what the total energy transfer is. It matters how much energy is transferred to the occupant's body. – The Photon Nov 27 '18 at 19:32
  • I can see this limit similar to being in a steel tower, as long as the deformation does not kill you. From what I have seen of damaged cars, the less deformation the better for the passengers, as the deformed stuff could impale the sitting driver.. – anna v Nov 27 '18 at 19:32
  • @annav, in the 1970's that was a huge problem. Now most passenger cars are pretty good at letting everything except the passenger compartment deform dramatically before the occupants are affected. In some cases (pickup trucks, for example) the design might not be so great, even today. I'd expect the car to start moving despite the brakes if the impact is great enough to fully deform the crumple zones. – The Photon Nov 27 '18 at 19:35
  • @ThePhoton But how did you determine that reducing the total energy transfer would not be the most beneficial way to minimize the impact to the occupant's body? Cars are designed to crumple in impact regardless of the speed; so I don't know why you would assume it is better to minimize net occupant movement with the trade-off of increases collision energy. Sure, your best case scenario is peachy; but it seems pretty obvious that it's an exceptionally ideal case and cannot simply be extrapolated to this situation. – JMac Nov 27 '18 at 19:39
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    @ThePhoton A head on 40 mph hit, I'm pretty darn sure your brakes and crumpling are definitely not sufficient to absorb the impact. You would definitely still skid regardless. – JMac Nov 27 '18 at 19:41
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    @JMac, because what was asked was how to minimize injury to the driver, not how to minimize damage to the car. – The Photon Nov 27 '18 at 19:41
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    @ThePhoton Yes; but your reasoning for why it would reduce impact to the driver is exceptionally weak in my opinion. It might point to some essential problems with this question, as this is highly dependent on the vehicle engineering, and not just what the cars are doing on impact. I don't think it's good to make a blanket statement that there would be less injury with brakes applied, because we cannot account for the excess energy properly due to how vague and hypothetical it is. – JMac Nov 27 '18 at 19:45