.... I live in an area popular with weekend bikers and there are numerous bunches of flowers on the verges of the roads to indicate where tragedy has happened.
Me too! Unfortunately many ride on the public roads as though they were on a race track. Many have crashes all by themselves but quite a few involve other road users (bikes and cars). I have always said that any driver/rider has the right to expect that the vehicle coming (perhaps even actually out of sight behind a curve or other obstruction)is doing something in the vicinity of the speed limit. That is to say not two or three times the actual speed limit.
Outside of the villages around the lake the limit is still 50 kph but everyone does 70 kph, that is reasonable, but speeds of over 100, even in the villages, are not unusual for these weekend warriors.