Straying a bit off the original topic now, but into the interesting subject of extra signals with lights.....
UK highway code is quite clear, flashing the headlights should only ever be done as a warning to let another vehicle know you are there, and likewise should always be interpreted as such, reality is quite different!
- Trucks here will generally flash their lights when an overtaking truck gets it's rear end clear of their bumper to let the other driver know he can pull in (seems way too close to me!), a lot of them will do it for the sprinter too and some will even do it for cars. I always try to remember to do it for them when the situation is reversed (when a main road has a reduced speed limit I tend to stick with it, when the limit is 50 most trucks will keep going at 55 or more so this happens more than might be expected)
- Another common use is at junctions etc. to let another driver know you intend to let them pull out in front of you. I get this a lot leaving the industrial estate I work on because I have to make a right (think left if you drive on the wrong side of the road) onto a main road where trucks and buses need to make a right off the main road into the road I want to emerge from, the main road is not that wide so they need to cut the corner, which means they need to stop short of their turn point and let the first car out before they can turn in, they flash their lights to confirm it. This is actually problematic if you follow the rules because you are supposed to interpret that as a warning not to pull out....
- If someone sees a problem with your vehicle they will often flash to let you know something is wrong (check you have your lights on if it's dark), the problem with this is that on some of our bumpy roads the car behind can often appear to be flashing due to change in headlight angle as the car is pitched up and down by the bigger bumps, which can lead to drivers getting paranoid and pulling over to check.
- Certain kinds of drivers, you know the type who drive fast German vehicles or white vans, who don't have any regard for the speed limit have developed a dangerous habit of driving right up close behind a slower vehicle and flashing the lights to intimidate them to move out of the way - often you are already doing 70.
- The last one I can think of for headlights is that having passed a speed trap or police car, some drivers will flash at oncoming traffic for the next mile or so as a warning. This might also be used to warn you of an obstruction or traffic jam coming up, or heading to my parents house for any of sheep, goats, or deer in the road.
The other main light signal used here is after you have let someone pass, or flashed to let them know it's safe to move in, as a thank you signal, after cancelling the turn signal (if they used one) they will flick the stalk one way then the other and then back to centre to give an alternating flash by way of a thank you.
All of the above is non-official and if you were to crash as a result of any of the signals the courts view would be that you should only have interpreted flashed headlights as a warning that the other vehicle was there.
I find it fascinating the way society has found so many ways to intend and interpret a flash of the headlights!
Do you have any different ones in other parts of the world?