Final answer:
To signal slowing down or stopping, extend your left arm out and move it up and down in a slow pumping action. This signals drivers behind to prepare for your reduction in speed or an upcoming stop.
Step-by-step explanation:
The hand signal you should make if you are trying to slow down or stop while driving a vehicle is to extend your left arm out of the window, with your palm facing down, and move your arm up and down in a slow pumping action. This motion alerts the drivers behind you that you intend to reduce your speed or come to a stop. Traffic signals also play a critical role in controlling vehicle speed at intersections, as vehicles cannot speed through the signal and instead must slow down and stop accordingly.