If a heavy car collides with a light car head on at, say, 60mph, the heavy car might slow to 20mph whereas the light car might be pushed backwards at 20mph. So one car experience 40mph of velocity change in a short time interval, whereas the other experience 80mph of velocity change.

This is hypothetical; I don't know what real numbers from real collisions are like, but light cars are definitely at a disadvantage in that they can get knocked around a lot more. There's also the possibility of a light car getting pinned between a heavy car and an immovable object or another heavy car.

I don't think this is zero-sum: ideally, we'd all be driving around in large bubbles that weigh 200 pounds or less and have huge crush zones filled with styrofoam or something.

> Is it really a zero-sum game? If a small car crashes head on with a large car, will the people in the small car suffer worse than if they crashed into another small car? To what degree?

Well it's zero sum in the linear sense as the relative total velocity change is the same, but crash survivability isn't linear. Rather, it decreases exponentially, since kinetic energy is proportional to the square of velocity - so a head-on crash between a semi and a compact car subjects the occupants of the compact car to nearly 4x the force that they'd be subjected to if they crashed into another compact car.

It's hard to disagree with physics.

However, please read what I wrote here:

https://news.ycombinator.com/item?id=23694479

To an amazing degree. In the limit, the large vehicle proceeds nearly unimpeded and the small one is a ball of tinfoil.

I mean, that makes sense if we assume that cars are sphere-shaped with consistent density. Of course, they aren't.

I guess what I was looking for was actual data, not assumptions.

Data? My son in his Ford F350 had a collision with a compact car that pulled out directly in front of him across the highway at speed. Small car: totalled. Truck: some scratches on bumper, loud noise.