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Comment by zdw | Hacker News Reader

There are so many ways this could have gone wrong...

For example, the guy was passed out - the initial impact could have knocked the other driver's foot off the gas, or cause it to press harder, further worsening the situation.

Also, if the bumpers didn't match, or if the angle of impact was not totally flat, you could easily end up in a "jackknife" style situation where the rear car pushes the front car at an angle causing it to loose traction and knocking both off course.

Not to write off his quick thinking, but I would be hesitant to try repeating this.

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Yes, indeed. Next time you should be very hesitant. Hesitate so much that the out of control car plows into a busy intersection. So many things could go right. It could just barely miss that SUV with 4 kids in the back, and I'm sure it would only scare the elderly couple crossing the street.

C'mon, seriously? The article is an up-beat account of how things can go right. How under pressure someone made a necessarily quick decision to take action rather than sit on the sidelines and watch as an observer. Do you really think anyone sane-minded individual is going to come away from this article thinking to themselves, "Man, next time I'm on the highway I need to give this a shot"?

If my math is right, the kinentic energy of 2 ton vehicle going 40 MPH is roughly equivalent to 70kg of TNT.

That's the kind of fire this guy was playing with, for his life, the life of his family, the life of the other driver and all the bystanders.

Not to understate his heroism, but had this gone horribly wrong he'd probably be a Darwin award candidate.

That's only really a problem if all that energy is transferred to another object (near) instantaneously - for instance if the vehicle hits a stationary object like a car waiting at an intersection.

Colliding at a relative speed of only a few miles per hour and then hitting the brakes allowed Innes to dissipate that energy in a more controlled way. Risky, certainly, but I don't think it's as risky as you think it is.

edit: as an analogy, think about landing a spaceship on a meteorite - it may be moving fast enough to wipe out civilisation as we know it if it hit Earth, but you could still safely stand on its surface. Relative velocity is key.

You wouldn't have to use a spaceship. "meteorite" means it's already hit the ground. You could just walk over to it, assuming it didn't wipe out all life.

Ah yes, perhaps "asteroid" was the right word? Large piece of rock in space :-)

If my math is right, the kinentic energy of 2 ton vehicle going 40 MPH is roughly equivalent to 70kg of TNT.

You're off by a factor of 1000. It's 62.9 grams of TNT: http://www.google.com/search?q=%280.5+*+%2840+mph%29%5E2+*+2...

Ah, my bad. I think the I missed the joule/kilojoule conversion in the reference I was using.

Thanks for the correction!

Not to rub it in, but this is one of those cases where a bit of intuition will get you a lot further than a calculation. Is it plausible that a 2 ton vehicle travelling at 40 mph is equivalent to 70 kg of TNT?

Think about the damage done by a crashing minivan. Enough to bend some metal, maybe even knock down a small brick wall. But a 70 kg TNT bomb would easily blow the car into many pieces.

Alternatively, how much fuel does it take to accelerate your van from a standstill to 40 mph? A tiny fraction of a litre, which is why you could do it hundreds of times on a tank of gas. Is that tiny fraction of a litre of gasoline going to be energetically equivalent to 70 kg of TNT?

> Is it plausible that a 2 ton vehicle travelling at 40 mph is equivalent to 70 kg of TNT?

I helps to know that TNT has roughly one tenth the energy density of petrol (4 MJ/kg v. 40 MJ/kg). If it took 7 kg of fuel just to accelerate a vehicle to 40 mph, vehicles would have to carry vastly more fuel than they do.

I pray I never fear to make the decision to save someone's life, all risks reasonably considered.

I have been in a similar situation, and I can tell you: your mettle is tested. You have a tiny amount of time to make a decision, so little that it almost comes down to your instincts. You have enough time to make an assessment of what kind of fallout is about to occur for all parties involved, and your main concern is, hopefully, to minimize the damage.

Sure, you may not have all the information at hand to make a perfect decision, and few of us can say to have any reliable precognitive power. You have what you always have in life: some information and the ability to make a choice.

Let not fear stay your hand. Fear kills as easily as a heart condition. It can always go horribly wrong, but when something appears by sound reasoning to be inevitable, will you have the courage to intervene?

Very true comment. I did CPR on a guy in the street a few days ago. I'm sure it wasn't perfect, but I'm now told that the guy didn't die.

http://blog.jgc.org/2010/10/you-never-think-youll-have-to-do...

In the OP the guy used the information he had to make a fast decision. The toughest thing in the situation I encountered was the moment I realized his heart had stopped and needed to decide right then to take action.

"Not to understate his heroism, but had this gone horribly wrong he'd probably be a Darwin award candidate."

It's not heroic if the stakes aren't high.

There is nothing wrong with this comment. The objections he raises are perfectly valid. Yes, it could have gone dreadfully wrong. In this instance, it didn't, and it is a credit to the engineer's skill that it worked. For many other people however, it would definitely be a bad idea to try.

Yes, it's great that he did it, and that he took fast action to save a life. It really is; it's a great story! But a down-to-earth comment is not worth downmodding over; it is neither wrong nor irrelevant.

If he matched the truck's speed before pulling in front of him, would it reduce the risk of the jackknife situation you described? Seems like it would to me.

Actually there was one other outcome possible if the cars made contact at a wrong angle (which was not that probable since the pickup was against a barrier and the engineers car was driving straight in the lane): The pickup would have pushed against the barrier some more that it would have probably stopped the car or at least reduce the speed at which it was traveling, while at the same time pushing the car in front to the opposite side, maybe making spin in place. If things went wrong in this situation at worse he'd have a car in a worse state and the man in the pickup could have died if the barrier made him topple over. The probability of the pick-up going into traffic again (which would make the car in front collide with the barrier, which could be fatal) was really small... hell uuid duplicate probability small at that.

not to mention "Without consulting the passengers in his minivan — "there was no time to take a vote" — Innes kicked into engineer mode."

more like "Innes kicked into irresponsible mode risking other people's lives to try something he probably saw on a movie"

Isn't that what governments do every day? They take choices for the well being of the country, without consulting me. Oh the irony.

That is kind of non-sequitur

It wasn't supposed to be. I was being sarcastic. Guess sarcasm doesn't translate well in the interwebs.

I think that saying 'Oh the irony' in a 'sarcastic' comment is far too meta to be understood in almost any context.

Come on. The risk was very low.

  1. Drive infront
  2. Match the speed
  3. Slowly reduce speed so that the car behind knocks
     into your back bumper at a relative speed of like
     1 mph. Maybe you feel a tiny bump.
  4. Apply brakes

Yes it's impressive, but I'd be willing to bet it's not that dangerous at all. The very very worst case scenario is that you don't match the speed very well and get rear ended, which is pretty much the best type of crash you can have.

Ways this could fail:

- breaks are not even across the wheels, he spins and goes right back into the traffic, guy behind falls over after driving the other way into the barrier

- collision affects one of the front wheels of the car behind, that car turns into the traffic - minivan either separates or gets locked due to collision

- engine in minivan stalls after collision, break assist stop working, simply pressing the pedal is not enough to stop both cars

- due to the shape of cars' front and back, the car in the front gets hit and one of the wheels get locked, suddenly breaks; the car in the back has enough kinetic energy to hit and injure passengers sitting in the back

- ... I could go forever

As in other posts above - I don't want to say it was stupid / bad that he tried... It worked, everyone's fine, it's great. But for me, my family in the car is infinitely more important than potential damage that might be caused by an unknown person in an car that's under no control.

You still seem to be missing the point that the impact velocity is like <1mph if you do it properly.

If it looks like it's not going to work, you just speed up again...

"But for me, my family in the car is infinitely more important than potential damage that might be caused by an unknown person in an car that's under no control."

I'm glad someone remembered this. Saving someone else might be a good thing, but saving your own family is definitely heroic.

I think I remember this being done on an episode of CHiPS.

Don't laugh: I learned how to shift w/o a working clutch from an article in Motor Trend and it came in handy one day.

It's not the same having a car hit you at 1 mph when you're stationary than when you're both doing 40 mph and the guy behind is unconscious. A number of things can go wrong.

The moment you have other people in your vehicle you're _responsible_ for their lives. Now everyone is cheering because it worked out but it could have easily ended up as a [Retard] tag on Fark.

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