I had an answer for you until I saw you are in Michigan! Oooooh Boy! Definitely contact your highway department or local government before you do anything or you may regret it. I know Michigan has some of the toughest environmental laws of any state. In fact they go way overboard IMHO. IMHO your DEQ (Department of Environmenal Quality) is made up of a bunch of (I won't say it on this board) but I've heard horror stories from fish farmers in your state.

Most likely you will need a permit if you can get one.

Be careful. I know of someone that was draining his pond in your state, and ran the overflow into a local highway ditch. A do gooder neighbor complained to the DEQ, and since he didn't have a permit he was fined $11,000. The water wasn't harming a thing. Some people just dont' have anything better to do.

Logistically what kind of distance are we talking from the pond to the highway ditch? From my experience I would either go with an open trench if your soil has a lot of clay to keep erosion down, or a smooth bore pipe. I've used the black drainage pipe that is corrigated and will never ever install that stuff again. Debris collects in the ridges over time, and just keeps on building until your flow either decreases or it plugs completely. Smooth bore pipe is much less likely to do that. Size of the pipe depends on how fast your pond backs up.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.