It is illegal for bicycles to be on interstate highways. Where there are no sidewalks (between cities and most of rural america outside the immediate town/city), there are signs that say "shared roadway" which shows a car and bicycle in the picture-because people assume bicyclists aren't allowed on the road. Where I live it is also illegal for bicycles to be on the sidewalk. Generally where the speed limit is higher, the shoulder is too narrow to accomodate bicyclists.

There is a very windy* mountain road that I travel frequently. It is also the only road into a tourist area, so bicycles and cars frequently share the road. Now, part of this road is on the side of a mountain on a steep incline, where a river winds next to the mountain. The shoulder is literally 4 inches wide, then there's the stone wall, then a cliff that drops into a river. There are also many blind bends in the road so you can't see if there's someone coming in the opposite lane. The speed limit there is 55. So, of course, people drive 70. If there is a bicyclist in the lane, you have to basically do a the sign of the cross and try to go around, or be stuck and add 20 minutes onto the travel time because there is now way to pass.

Much of this issue could be solved simply by widening the shoulder. As Aemon said: infrastructure. This is on backcountry roads of course. I don't mind bicyclists. Most of the time I am not bothered by them, and I always watch out for them. Especially when the speed limit is low, there is lots of stop-and-go traffic and pedestrians to watch for, etc anyway. However, there are many places in this country where it's simply dangerous (for cars and bikes alike) to have shared roadways. And I agree with Aemon's point in that respect. Many of our roads are specifically designed to just be for automobile traffic and don't include the proper shoulders/paths for safe bicycle travel. I'm not saying it's right or wrong, it just is.

*as in wind the watch, not wind that blows