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.
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.