We are talking race tires here. And as with anything race oriented, there are compromises to be made for street use. Drag race tires are more expensive, don't like wet surfaces, don't last nearly as long, etc. While I am sure that the 'not for hiway use' is butt covering 101, the tires can build up a lot of heat at sustained hiway speeds. I run short distances at hiway speeds with my DRs and know of a lot of other guys that do too and I have not heard of any problems. But running extended distances at hiway speeds is rolling the dice to some extent. It's just not what the tires were designed and built to do.