[drivebyhooting]

> What is the physical mechanism that would cause a vehicle hitting a more massive vehicle to be worse than the same vehicle hitting a wall at the same speed?

That’s not the comparison to draw. It should be why is hitting a more massive car worse than hitting a smaller car? The answer is in the physics of the collision and momentum.

Against a solid wall the advantage belongs to neither big nor small. The collision physics is identical, bar crumple zones and vehicle deformation.

That gets into my second quip about minivans. The rear row is unsafe since there is hardly any trunk.