If car seat belts were designed and tested in frontal impacts at up to 20 mph, then would you believe that they would make a difference when traveling at 60 mph?
Because you're basically saying that bike helmets would make a difference in a collision scenario where the impact force would far exceed their design and impact testing standards.
Yes, absolutely I'd rather be wearing a seat belt designed to to a 20 mph spec in a 60 mph crash than no seat belt at all. I'd take anything at all to slow down my body/brain from coming to a complete stop.
The sudden stopping is what kills. If the only thing slowing down my brain is my skull then that is worst case scenario, unless of course, my skull shatters, then I guess my brain slows down at a more acceptable rate as it spills onto the road.
I'll take anything at all between my brain and a hard object trying to rapidly impart some force unto it. A helmet does this very well.
> I'd take anything at all to slow down my body/brain from coming to a complete stop.
Except that in this case, the belt just comes off its anchor points and doesn't slow you down at all. Just like a bridge rated for a 10,000 lbs load will collapse when a fully loaded 80,000 lbs tractor trailer drives over it.
If something is going to work, then it had to be designed and tested for it. You can't simply believe it's going to work in situations that it wasn't designed and/or tested for.
Let's say you're on the 10th story of a burning building and the fire department is below telling you to jump. They have one of those cartoon sheets to catch you but it's only rated to catch people falling from the 5th story. If you jump you'll hit the sheet at 40 mph but break through it and hit the ground at 10 mph. Do you shout at them to take the sheet away because you want to hit the ground at the full 40 mph because the sheet won't do enough good?
Edit: and just to address this specifically
> Except that in this case, the belt just comes off its anchor points and doesn't slow you down at all.
This is not true because the energy that went into breaking that anchor point is energy your body is no longer carrying. The belt did slow you down and even in breaking could still have saved your life.
Because you're basically saying that bike helmets would make a difference in a collision scenario where the impact force would far exceed their design and impact testing standards.