There is a much safer, but less obvious, way of waving your thanks to other motorists that is used in France but not in the UK: Leg Waving.
When, for example, (as a car driver) I pull in to let a bike go past, UK bikers often raise a hand in thanks. This leaves them one hand short on the controls and, should anything happen in that short time, (a big pothole or sudden manoeuvre from another vehicle) the bikers response time is much longer as you need to get both hands back on the controls before you can do a great deal.
On the continent, bikers have come up with a great alternative. The simply straighten out the leg on the side which we UK peeps would normally wave. This means you still have full control of the bike, delaying only the ability to change gear or apply the rear brake. Much less of an issue than a hand off the bars, in my opinion.