hand-signal the driver to stop
give hand signal for the driver to stop
To hail a car on the road.
"Hail" is a gesture in order to stop it.
In BrE, you would only hail a cab (we also use "flag down"). It doesn't work for stopping a normal car on the road. If, for example, there were an accident ahead and you were trying to stop the traffic coming up behind it, there wouldn't be a standard verb for it. I might say "I stood in the road and frantically waved my arms to stop the oncoming cars".
They waved down a passing car.
He just pointed to the road? I would expect a sweeping gesture to the side, to indicate that you should pull over.
I think I reacted the same way most Americans would when I saw this: I looked at the officer, slowed down a bit, and drove by. An American cop would make dramatic gestures to convey the message "pull over, now!"The purpose of stopping the car decides the context and what gesture you should use.