I'm not sure that is the correct meaning, though I don't claim to know anything about Texas law. Years ago while touring in Colorado, there was a similar rule. There was a short stretch I rode on the I-70 shoulder (in the vicinity of Silver Plume, I believe) where it was literally impossible to drive or bike to certain towns any other way. As soon as there was some sort of frontage road, however, it was illegal to bike on the I-70 shoulder. Given that Texas doesn't have anything like the narrow mountain valleys of Colorado, if the law is interpreted the same way, there would be very few places in Texas that you could not reach by something other than an interstate highway. Anyway, riding on interstate shoulders is not fun.

10 Wheels, your map as well as your averages are including the presumably hundreds of miles Ryan traveled in a car to get to Phoenix. Even though MI007 conveniently omitted that, Ryan, to his credit, said that he went to Phoenix by car. Where he got in the car, however, appears to be a state secret.