We're talking about filming major roads here. "Motion activated" effectively means running 24/7. No way a built in solar panel in a camouflaged trail camera will manage that. Also it doesn't matter how small the device is, if you legally want to attach a camera to any sort of infrastructure overlooking a road, then yes you need permits you almost certainly will not get. You could possibly attach it to a pole on private land next to the road, but even then you'd need the permission of the land owner.

A random solar powered android phone is found fastened to a bridge and you can be sure the police are going to look into it and take it down.