Just posting the California law. It is amazing how many adults I know that don't know hand signals. They don't use them as drivers after obtaining their licenses and must re-learn as cyclists. Because it is safer for me, I just assume that all drivers don't know hand signals. Since I also get to experience college students en mass as commuters, I also assume that any cyclist in street clothes doesn't know hand signals or probably doesn't care. Matter of fact, it frequently appears that they don't think common courtesy and vehicular rules of the road apply to them.
http://www.dmv.ca.gov/pubs/vctop/d11/vc22111.htm
Hand Signals
22111. All required signals given by hand and arm shall be given from the left side of a vehicle in the following manner:
(a) Left turn-hand and arm extended horizontally beyond the side of the vehicle.
(b) Right turn-hand and arm extended upward beyond the side of the vehicle, except that a bicyclist may extend the right hand and arm horizontally to the right side of the bicycle.
(c) Stop or sudden decrease of speed signal-hand and arm extended downward beyond the side of the vehicle.
Amended Ch. 751, Stats. 1976. Effective January 1, 1977.