Hell no.
Most importantly, if you're hooked up to a mainframe, then someone will have knowledge of your daily routes and whereabouts. Not that I personally do anything that I need to keep hidden, but the lack of privacy involved inhibits freedom to an extent that I'm not comfortable with.
Also, if there's people out there that can hack into secure gov't sites and the like, then what's to stop someone like that from fucking shit up with the right motivation? Seems that the hacking of a highway mainframe has the potential to cause great damage.