Here's one pure and simple. 5 rapid Highbeam flashes..
And then one less for each generation, 1-Gen1 to 5-Gen5.
Or we could go with two flashes like mentioned before. I dunno, but I think HB flashes would be the safest bet (unless you're in the inner-city.) It may seem dull to some, but I find the whole thing novel, also having ridden a motorcycle and knowing that MANY bikers do their own wave to show that motorcyclists are a community for whom each have much mutual respect for one another.