Joined this site just to post a reply in this thread.
I am a vespa rider, an old school px200 with manual gears (on the left hand). The only time I wave to scooter riders is on the open road, in town it's usually just some kid in shorts, a t-shirt and flip flops and if you wave at them they get distracted and plow themselves into the back of a parked car. On the open road on the other hand I do wave to other scooters, usually because they aren't ridden by kids in flip flops, usually they are larger scooters ridden by people like myself who love the vespa style, or have ridden bikes and actually know what the hell a wave is!
I wave to all motorbike riders on the other hand, and get about a 99% return wave rate. I like to think that this is because I have a real scooter, old school with gears that reminds people of why scooters are fun to ride in the first place.
For some reason in the sates it sounds like HD riders never wave to scooters but sports bike people tend to a little. In new zealand it's kinda different, HD riders are the ones who wave to you, see what your riding and turn around to chase you down to spend 30mins asking you everything about the scoot. Sports bike riders on the other hand are too busy hanging on for dear life and trying not to get themselves killed. Never had a HD rider not wave back to me, most sports bike riders over here just give the "nod".
But really, in the end of the day, we are all on 2 wheels, we are all riding an inch from death, and we are all enjoying ourselves and having a good time. Some people like scooters, some like HDs, some like sports bikes, they are just different flavors of the same milkshake.
