What hand signal does a policeman give to indicate that a vehicle must stop?

The hand signal given by a policeman to indicate that a vehicle must stop is the arm raised straight up. This signal is universally recognized as a command for vehicles to halt. The extended arm positioned vertically communicates the need for immediate attention and compliance from drivers, indicating that they should come to a stop.

When a police officer raises their arm straight above their head, it creates a clear and direct message that is easily visible from a distance, enhancing safety by reducing confusion among drivers. This is especially crucial in traffic situations where clarity and prompt response are essential for maintaining order and preventing accidents.

In contrast, the other options do not convey the same immediate urgency for stopping a vehicle. For example, arms straight out to the sides usually indicate a directive for vehicles to proceed with caution, while waving a hand could signal various actions such as directing traffic or allowing vehicles to move. Pointing downwards typically does not indicate a stop and might suggest something else entirely. Thus, an arm raised straight up effectively fulfills its purpose as the appropriate signal for stopping.