Journeyman:
I'd report that to the police tbh. Might sound like an overreaction, but that sort of dumb crap can cause a nasty smash on the motorway.
Funnily enough as we rounded the bend about 5 seconds later there was a bike cop stopped giving a ticket to someone on the central divider. Would have been some delicious irony if he had witnessed this incident and busted him red handed.