I don't think it would be much of a problem in Vermont, but in parts of California, getting off the freeway at the wrong place can be more hazardous to your health than staying on it...
Further, I would be careful about dropping anything on the roadway. Yes, you may slow them down while a few ball bearings bounce off their windshield, but in the long run, that will just provoke them to be more aggressive towards you. On top of that, the debris that you drop today could be kicked up by a car and fly into your visor tomorrow ...
The company Victorinox (yes, the company that makes the Swiss Army Knives) sells a knife called the "Rescue Tool". (
Victorinox - Multi-Tools - RescueTool ) I'm thinking about getting one of those, and putting it in the side pocket of my tank bag. The "Rescue Tool" includes a hardened point to break windows in case of emergency. If provoked, I would drop back until I was close to the driver's side window, then tap (shatter) it and then open the throttle before the driver can even react.
FWIW, the one-handed version is only available if you have a weapons permit or are a member of a fire department or rescue organization. My fire department can't afford to buy these right now (they cost about $80 each), so I may just buy the "normal" two-handed version for myself. The window breaker tool is on the end of the knife, so I don't need to open anything to use it.