I blame the American.
That is American (singular), not the entire population of the United States of America. Mind you, quite a few of them could be described as most singular persons! But no, the singular American whom I blame is, possibly, the most Anglophile of them all, Mr Bill Bryson.
And for what, you might wonder, do I blame him?
Read on, and all will be revealed.
(No, I do not intend to perform a strip tease. That really would be most unedifying!)
Put simply, what I blame the estimable Mr Bryson for is the Yorkshire wave.
There was a time when a car driver wishing to thank another driver for giving way would simply flash his headlights, a simple manoeuvre that can be performed quite safely without taking either hand off the steering wheel. Well, most people can reach the lever while still holding the wheel - although the Old Bat claims her fingers are too short. but (apart from fingers that are too short) there is a snag. If used incautiously, that headlight flash could cause confusion with the other driver thinking he was being allowed to take priority. Nowadays, most drivers simply lift 1, 2, 3 or 4 fingers from the steering wheel.
And how does Mr B come into all this? It was he who coined the phrase 'Yorkshire wave' when, in one of his books, describing the laconic way in which dour Yorkshire farmers greeted people they recognised while driving. they would, claimed Mr B, simply lift a single finger from the wheel by way of greeting.
The acknowledgement that most drivers give to others now is simply an extension or development of the Yorkshire wave, which was quite unknown to we soft Southerners until we read about it. And now it's seen everywhere!