Cutting with a rotary cutter along a highway is insane. That machine can easily hit anything thrown out of a car and send it through a window or windshield like an artillery shell and kill someone, or even someseveral people.
Flail mowers should always be used when people are around. Light duty, which have knives, are for turf. Heavy duty, which have hammers to break stemmy material, are appropriate for this roadside job. A flail will not launch missiles at people and cars. When people and easily damaged property are not present, a rotary mower or cutter makes more sense than a flail. I am trying to avoid using inappropriate language here, but I cannot emphasize enough how bad of an idea it is to use a rotary cutter along a highway. You are not only putting yourself at risk of being sued for every last penny you and your family will ever see, you could easily kill innocent people. You may as well be doing target practice with the targets on one side of the highway, you and your gun on the other, while using a gun which has random time delay that you can never anticipate.