Ah, headlight flash... could mean so much. Some drivers use it to say hello when they meet another driver from the same company. But they're told not not.
I once met a guy that flashed the headlight. "Hello" I thought and saluted him with the left indicator. After passing him I saw a car standing across my lane...
That was what he meant with flashing the headlight... to warn me about the car...
Often, flashing the headlights means "You m*ron" or something like that. But it can also mean "Hey, I've stopped here, so you can pass the doubleparked car"
By the way, in Sweden you must have lights on even in daylight.
I often drive in the dark. Nice to drive on a dark road, in rain and meet another bus in a 90 degree turn
Western SMT post about canada can apply to Sweden, except the "after you"