The Adventure Cycling Association has worked with the American Association of State Highway and Transportation Officials (AASHTO) to get a nationwide bicycle route system approved.  The system, to be called the U.S. Bicycle Route System (USBRS).  The map above is the existing Adventure Cycling Route Network, which links together rural roads to create a low-traffic route through some of the nation’s best scenery and historically significant points.  The USBRS would likely incorporate most or all of this, but expand it to approximately 50,000 miles of designated bicycle routes (up from the current 38,158 miles).
Get the full story here, with lots of relevant links, and check out the Euro equivalent, called EuroVelo, here.