According to this link, it's legal to ride on the Interstate (shoulder) in Texas as long as there is no alternate (meaning, you can't do it in an urban area):

http://www.crazyguyonabike.com/doc/?...oc_id=2616&v=v

I'll update if I get other info...

Also, it looks to me like you could save about 170 miles of Interstate riding if you got *off* of I-10 where it intersects with highway 190. Then take 190 east until it hits highway 83, follow that down to Fredericksburg and on in to Austin.

It's only about 20 miles longer, and saves you two days of riding on the Interstate.