In short, turn or stop hand signals, historically come in two styles: British and American.
The British way is (sitting on the right side of the car):
- to turn right, you stick your right arm straight out,
- to turn left, you stick your right arm and rotat it counterclockwise,
- to slow down or stop, you stick your right arm with the palm down and move it up and down.
On a moped, motorcycle or bicycle, the left turn signal is to stick the left arm straight out.
It used to be common (and legal), to signal turns with a whip. To slow down or stop, you'd hold your whip straight up and turn it counterclockwise. To turn left or right, you'd do the same thing and then tilt the whip to the side you were about to turn. This system was completely abandoned in the 1950s.
The American way is (sitting on the left side of the car):
- to turn left, you stick your left arm straight out,
- to turn right, you stick your left arm out and bend your elbow so your forearm is vertical upwards,
- to slow down or stop, you stick your left arm out and bend your elbow so your forearm is vertical downwards.
On a moped, motorcycle or bicycle, you can also signal a right turn by sticking your right arm straight out. In most U.S. States, its not legal to drive a right-hand-drive vehicle if the turn signal lights aren't working, because you can't make the correct hand signals.
Japan (drives on the left) uses the American signals but with the right hand.
The British system is commonly used in the UK, Ireland, India (theoretically), and in most of the former British Empire, except New Zealand (Japan left turn signal = stop or slow).
The American system is commonly used in the USA, Canada and most of central and south America (sometimes theoretically).
In Europe, signalling varies country by country. Germany have not hand turn signals in Rule of Law and hand turn signals are forced for horse drawn carriages, bicycles and mopeds drivers by Case of Laws of Federal, or States, courts. If a car has broken turn or stop lights, you can not drive it. In the Czech Republic, Slovakia drivers use American-style hand signals only if they have no turn signal lights (trafficators are not allowed), or if they are broken. Stop / slow down signal is not fixed. Some states have different hand signals (American left turn signal = stop, and so on).