In Montana, you can get light kits and plates for ATVs to make them "road legal".
That said, folks drive them on the county roads and sometimes along the hiway without such niceties and the cops don't bother them much AFAIK.
Our neighbor for instance, has about four miles of county road running through his ranch - open range, with cattle guards at each end. At 90, he's gotten a little too old to ride a horse anymore. But the sheriffs don't patrol up here anyway :rolleyes:
I think if you want to get technical, that you can tie a red triangle onto the back of most anything and you are legal as long as you stay under 25mph. Though you hardly ever even see the triangles on tractors and other equipment. If you can't see a 150 HP tractor with a folded up disc creeping along the hiway, then you ought not be driving anyway
Neither do I have license, lights, etc on my horse when we have to ride along the road - though we have jumped into or across the ditch in a hurry a few times