Here's how bus and truck drivers in Vietnam communicate and warn each other about traffic police
Making and releasing a fist is one way to ask other drivers: “Did you see any cops?”
Waving one’s hand left and right means: “No”
Quickly flashing the headlight is another way to ask other drivers on the road if there are any traffic officers ahead. But the signal also means other things. If a driver does it at night, they mean to tell their colleague on the opposite direction that they are keeping their headlights on high beams. If they do it during daytime, they are warning others drivers of some dangers ahead.
Pointing with two fingers is to make a gun to warn other drivers that there are traffic police with radar speed guns ahead.
This number 1 means there are traffic officers ahead, but far away.
Thumbs down mean cops are right in front of them.
Drivers apologize to each other with a two-finger V.