Traffic Hand Signals

The practice of using hand signals in traffic is largely obsolete. Modern motor vehicles use electrically-controlled lights to signal a turn or stop. These signals, originally intended for general use and still taught in drivers' education courses, are now most often used by cyclists, and occasionally by motorists whose vehicles aren't working properly. I'm assuming here that traffic is flowing on the right side of the road and the driver's seat is in the left of the vehicle, as is the case in North America.

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Stop

Extend your left arm out the driver's-side window and bend downward ninety degrees at the elbow.

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Also, qousqous mentioned that a cyclist can signal a right turn by fully extending their right hand.

As a cyclist I know and use traffic hand signals as a habit while riding, blading, or otherwise following the flow of traffic. I find it disconcerting how few others are able to interpret them: even other cyclists who should know better end up just pointing to where they are going to turn because so few motorists comprehend the signals.

To sate my curiousity I checked in the driver's instructional manual for the State of Massachusetts and confirmed that hand signals are described. The incredibly low requirements for attaining such a license do not ensure that most actually learn or retain the information: a regretable state of affairs.

Are traffic hand signals are destined to follow Morse Code into functional trivia?