It costs nothing to do and if one acknowledges a scooter rider they may start to think that bikers are OK after all, rather than muttering "ignorant cnut" under their breath as you pass by, forcibly ignoring them. I'll normally acknowledge any other two-wheeled road user no matter what they're on - Harleys, dirt bikes and scooters included. The only time that I'll not make a point of doing it is when I'm engaged in a manoeuvre of some kind; braking, passing, cornering and the like, and I think that I'm quite justified in doing so without appearing as aforesaid "ignorant cnut". Most riders acknowledge, we must have done 80-odd miles this afternoon and I don't recall being blanked by anyone - not that I'm especially bothered when they do, it's their choice (ignorant cnuts).
What does make me laugh is the super enthusiastic signallers, waving and gesticulating frantically like a shipwreck survivor seeing another vessel for the first time in years.... normally newbies, bless 'em.
