JESUS! (multiple) -- I like how this one can be expressed to match the mood. A kinda soft "jeeeesus" if you're boggled by the stupidity of the driving around you or a really sharp, sweary "jesus!" if you're pissed off, etc. But really, all of these would have been improved if they'd been accompanied by a soundtrack.
Eat our 72oz steak in an hour and it's free! -- I can think of a lot of ways to say, "no, thanks," to that one.
"Adult Superstore!!!" followed about 50 feet later by, "porn ruins lives!"
Really quite a lot of adult video superstores. Wow.
In OK, an advertisement for vasectomy reversals in Houston (appx 9 hours away).
"Do not drive into smoke" -- these were posted every few miles on the interstate in OK. It seemed like it would be the kind of obvious common sense that doesn't need stating, but apparently, they were necessary.
Speed limit 17 -- ?? I guess 20 was just TOO fast, and we all know no one's gonna go 15. Or something.