Nice idea. If you get buzzed too often, by all means use the flag. I couldn't use it here due to too many other cyclists, and I wouldn't because there are narrow, slow streets where I really don't mind cars passing me with less than 3'. But being fine with 1.5' at 10 mph delta (me 20 mph, car 30 mph) doesn't mean being fine with 1.5' at 60 mph!