Vehicle mass is a significant factor in collisions between vehicles because taking two thirds of the impact is a meaningful increase over taking half of it. Heavier vehicles are much more dangerous to lighter vehicles.

It's basically irrelevant in pedestrian collisions because the pedestrian is already taking ~95% of the impact even from a lighter car and the <5% difference between that and an arbitrarily heavier vehicle is completely dwarfed by any of the other variables like the speed of the collision. For a pedestrian it's better to get hit by a freight train at 35 MPH than a 3000 pound vehicle at 39 MPH.

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seizethecheese 9 hours ago | parent [-]

I was thinking of F = MA, where mass seems important here. Is this the wrong mental model?

Edit: okay I get it, the human acceleration from impact basically asymptotes.

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AnthonyMouse 9 hours ago | parent [-]

You're solving for force when the thing that causes the damage is (very rapid) acceleration. The total acceleration comes from the initial difference in speed between the two things that collide and the proportion of the acceleration applied to each object is inversely proportional to their relative masses.

If they have the same mass then they each take half. If one of them is 19 times heavier then the other one takes 95% of the acceleration. If one of them is six hundred trillion times heavier then the other one takes ~100% of the acceleration, but this is still only ~5% more than taking 95%.

Consider what happens if you walk at 2 MPH into a concrete pillar which is fully anchored into the Earth. The mass of the Earth makes the mass of your body look like zero with some zeros after it, so are you immediately crushed? No, but get launched out of a cannon into the pillar at 200 MPH and you're dead even if it's not anchored into the Earth.

	▲	seizethecheese 7 hours ago \| parent [-]
		> Consider what happens if you walk at 2 MPH into a concrete pillar which is fully anchored into the Earth. Excellent thought experiment and this helped it click.