The younger people here might not know about this, but long ago there was a universally understood–well, at least in America AFAIK–system of turn signals that you made by hand, through your open window. To signal a left you stuck your hand straight out to the side, usually with the palm facing downward. To indicate a right you stuck your hand out as before, but bent at the elbow with your forearm held upright. And those were the only two I ever learned. They were still teaching this stuff when I learned to drive in 1974. IIRC you were required to use signal lights at night, or in conditions of poor visibility, which, in those days, were usually the same time you’d rather have the window closed anyhow. I think they may have been phasing this out when I took my first official driving test in '74, because the tester didn’t ask me to demonstrate the signals.
This was in California. Did they have something similar in your state or country? Presumably if you drive on the left the signals described would be reversed, or something.
For some reason I just wondered about this today. I hadn’t thought of it in decades, and I can’t remember the last time I saw anyone do this.