It's a little tough answering this question, since you didn't specify which patrol you were asking about. Therefore, since I see you're in Maryland, I'll answer in a general way for that state;
Most major highways are the responsibility of the Maryland State Police. They, in general, are using a 39 mhz simplex system; however, many barracks are now getting on their respective county's trunk system via the use of pre programmed portables.
Some roads, such as those leading up to tunnels are the responsibility of the Md. Dept of Transportation; either they or the Maryland Transit Authority.
You can find the State Police layout, along with the MdTA and MTA frequencies in the Maryland databases on this board.
73s Mike