I've noticed this weird driving etiquette in Providence: if someone is trying to turn left and there's an unending stream of on-coming traffic, some driver will invariably slow and flash their brights so the left-turner can sneak through. Actually it goes for right-turners, too, and buses, and pretty much anyone caught in a tight spot.
This city has some serious traffic-flow problems -- lots of one-lane streets, wonky five-street intersections, and chutes-and-ladders interchanges -- so it strikes me as a creative and uncommonly civil solution to give a fellow driver an extra inch to squeeze in.
There are plenty of assholes on the road, don't get me wrong (high on my pet-peeve list are those who sail through red lights just because they feel they've been waiting long enough), but the courtesy of other drivers trumps them. Incredibly, it's the perfect antidote to road rage. I've actually started looking for opportunities to let people in, and I feel bad when I miss them.
I have a head cold and feel like I'm wearing a diving bell, which may be why I'm writing about traffic and seriously contemplating further research into it.
Carry on, then!