The real problem is explosive population growth. While increased road capacity does eventually fill up again, it's not because individuals drive a lot more, it's because of population growth. If we widened our roads (as we're going to have to do) but kept metro Denver population from growing to 4M ( then 5M, 6M...) we'd be just fine. We'd have good traffic flow.  But people just keep pouring in to the front range, and into the country as a whole.  

We can't stop other citizens from relocating to CO, of course.  BUT, as a country we can and should be slowing our immigration quotas.  That's the real solution - or at least a mitigator that needs to take place IMO.  It wouldn't stop the relocation flow into CO, but it would slow the total flow.

Of course, our politicians will never agree to do that, so we're gonna be stuck with millions more flowing into the state in the next few decades. Even with transit expansion, we need road expansion. Just no way around it. Roads are too important.