What is the origin of the honk your horn gesture kids do to get truckers to honk their horn?

Question

What is the origin of the honk your horn gesture kids do to get truckers to honk their horn?

Answer 1

It goes back to steam locomotives.

It is the same arm movement the engineer used to blow the whistle.

Answer 2

In the older semi-trucks there used to be a chain hanging along the upper sash of the drivers windows. you would reach over and yank on this chain to HONK the air horns.  The gesture looks just like what the driver would do when he/she pulled the chain.

ps

(Its location probably did come from following the style of locomotives)