Restricted highways, in the US, are clearly marked. They're largely limited to the US Interstate system. At every on-ramp will be a sign banning bicycles, farm equipment and all slow moving vehicles.

Millions of US cyclists ride the highways. It's reasonably safe and much more enjoyable than the city. You can actually get some decent speeds since you rarely need to stop for anything.

And, as others pointed out, something demeaning about your being a child and a girl, etc etc.