Sometimes in those statutes it states: 'Roadway is defined as any improved part of a road which is designed for vehicular traffic' which could mean any asphalt/concrete, and even the graded part of the dirt shoulder, if they want to go that far. Best thing to do is look up specific laws in your state. I think HitchWiki has all the laws from almost every state for hitching, listed by state.
I've had this discussion with cops hitching before, and like others have said, I end up just giving in and saying I'll GTFO as soon as possible, because continuing my trip was much more important than making a point and going to jail. I won that argument once hitching in some random exit west of PHX in the I10, lol. Cop said if I was there the next time he came around he would take me to jail for vagrancy.