I did a cycling tour in Montana back in the '90s when Montana's speed limit was "whatever you consider to be safe" (or some legalese to that effect). Let me tell you, the first time a car passes you doing 100+ mph, it's an eye opening experience. The roads didn't exactly have shoulders either.

That said, there is NOBODY outside of the "cities" in Montana; we'd see maybe one car every 30, 40 minutes. We never had a problem (i.e., cars would go clear into the other lane) except with logging truckers. We became convinced that logging truckers received some sort of bonus based on how many people they kill.

I've logged my fair share of miles on roads with high speed limits. To me, it's not the speed limit, it's the volume of traffic. Occasional cars doing 60+ is no big deal, constant flow of traffic doing 30 is far scarier to me. I have no idea how people ride in cities.