I think the heavier vehicles was mentioned as a "safety" thing. As in if you're driving a 4000 lb suv and you schmuck into a 2100 lb honda, you're going to be in a lot better position than the other guy because you've got more momentum and kinetic energy, and thus you'll experience less violent decelleration. Similarly, if you get schmucked by a transport truck, you'll be a bit better off. As for anyone in a geo ... well, if they were rich, why were they driving a geo? And if they're not rich, why should america care, we're at war and omelettes and eggs and so forth. But I might be a bit cynical on that point.
I think the heavier vehicles was mentioned as a "safety" thing.

Yes, it has always been contended that CAFE standards have resulted in lighter vehicles, which in turn is blamed on more US highway deaths.  I don't have any studies (pro or con) about this claim, but it is frequently repeated in any discussion of CAFE standards.

How they can rule out things like speed, distractions (hey, was that my cell phone or my beeper?), substance abuse, etc. I don't understand.

I think that in order to be a candidate for congress one of the requirements (and I'd add quite a few) would be knowledge of what "post hoc ergo propter hoc" means. And heck, probably go so far as to actually have a 100 question test on logic. Make it elementary enough that a requirement 100% right wouldn't be too tough, but still such that it would show some argumentative skills other than rhetoric.