What I'm finding interesting is, let's say the car behind wasn't a police car, but someone who rapidly approached the car in the
way I suggested above. Maybe someone agitated that they're traveling much slower, so forcing them to make a decision by declaring their presence.
The guy might
still have interpreted that as an invitation to race, which is to say that he wouldn't have taken the back car's intent for what it was meant to be. The dude behind is agitated and suddenly sees a response, but not because the dude in front was threatened.
Basically I'm just seeing a case here where a possible intended IM ends up being read as something completely different by another person, but the end result ends up being, I guess something in which in this hypothetical the participants are somewhat "positive" -- the guy behind got the guy to move, the guy in front wants to race.