All around East Coast side of USA I have seen them.....especially in regions and Counties that have large rallies and events...and of course where there has been loads of accidents involving Motorcycles.....
Here in UK we have them everywhere that there has been more than 2 Motorcycle accidents in a 2 or 3 year period...that means ALL OVER UK.....
A couple of roads I use here have signs up that say "68 Motorcycle Accidents on this road in the last 12 months, 5 fatalities, LOOK OUT FOR BIKES!!!!!