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Comment by mwint | Hacker News Reader

What’s the argument for mass being a driving factor in pedestrian safety? You’re outweighed substantially by anything bigger than a moped. Certainly anything bigger than a Civic. Seems like it’d be more about body angles, visibility, and driver attentiveness.

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Outweighed substantially is doing a lot of handwaving. Turns out that difference matters[1] when it comes to injury.

Maybe an analogous question would be: would you rather be stepped on by a horse or an elephant? Both substantially outweigh you but I bet you'll pick the horse.

[1] https://www.nber.org/digest/nov11/vehicle-weight-and-automot....

If we're talking cars hitting you at 80mph, sure you're probably dead no matter what. But a 9,000 lb car hitting you at 35mph is traveling with the same momentum as a 3,000 lb car going 105mph. You might survive a 3,000 lb car hitting you at 35 mph, you're unlikely to survive the other.

This feels... wrong, somehow - we wouldn't say that a 100,000lb shuttle transporter hitting you at 1mph will do the same damage as your 3,000lb car at 35mph. There's way more dynamics at play here.

I agree. If I jump up and down and impact the earth with my feet it's quite harmless. I suspect above some lower bound it doesn't matter how much something weighs, it's all about how fast something hits you (because it's mostly you that's being accelerated).

Although the weight (and correspondingly, the size) of a car does play into sight lines, maneuverability and how fast the car can decelerate.

It is kind of wrong. What matters is the force impacted upon the person (their acceleration), not momentum. Basically, change in velocity over time is what really matters, but because momentum is conserved, it influences what the total change in velocity is.

You are right I think, it's about impulse, force over time, surface area and how much energy can be transfered through the surface area through the underlying tissue and create a ripple basically. I suspect that at some point, the 'transfer function' of energy just maxes out and mass of the vehicle itself is pointless over that point, as it's all lethal.

If you're at a crosswalk, then you'll roll over the hood of a Camry or Mustang going 30mph.

If the same thing happens with a Escalade or F150, there's more mass and it's hitting dead on.

Bumper height is the factor there, not weight. A higher bumper is more likely to knock you down and under, not up and over.

Brake distance, the heavier the vehicle the longer it takes to slow it down.

I'm surprised I'm not finding a weight vs. stopping distance graph anywhere. I know that the physics theory says weight doesn't matter for stopping distance (spherical cow in a vacuum), but there's some nonlinearity in the friction response that does make it matter somewhat. It'd be interesting to know how big a different it does make.

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