See: the disparity between road quality in Maryland (good density) and Pennsylvania (poor density). (or, to correct for climate, compare Pennsylvania with New York).
I'm certainly no expert on the topic, but I haven't heard any news about any major problems with Australia's roads. I'm curious to know how they compare with roads in the US.
USA density: 33.7/km2 87.4/sq mi population: ~ 314 mil
AUS density: 2.8/km2 7.3/sq mi population: ~ 22 mil
The roads in America are horrible by Australian standards. When I bring friends to the US they are shocked and say things like "it just doesn't look finished". This is more-or-less true for the majority of social services in America.
Of course, the climate in Australia lends itself to much better roads, but in comparison to Canada's roads, America's still suck, so there is no excuse.
I'm not saying that states with bad roads don't have themselves to blame, just that density is a factor.
You're saying that it's got more to do with how much the population is willing to be taxed, which I believe is the real reason.
I guess what I'm saying is that while dodging the real cause of this issue, Americans are going to point to large geography rather than large population as the 'root cause'. In this case that isn't entirely without merit, since for a lot of Americans (those in Maryland for example) the '(tax rate * population) / area' equation works out alright.
One summer when I was living in Philadelphia they removed the surface of the road in front of my apartment. Two months later they put it back. And if it's not that, it's crews filling in holes from road-work with about half as much asphalt as the hole needed, or the random patches of cobblestone street still left in the city, seemingly with little rhyme or reason.
Now, I think that's a pretty fair comparison. The roads can be pretty terrible in Washington and Oregon from my experience. That may be because federal money for roads goes to other less dense states, but these proportions hold up fairly well nationwide too. I think it's just a matter of less public funding, it's that simple.