You are driving on a two-lane highway and are passing a large truck. If the driver of the truck blinks his lights, it means

A: It is clear for you to pull back in.

B: You must not pull back in yet.

C: The truck driver sees something wrong with your vehicle, such as a flat tire.

A, lane change clearance

Explanation:

Truck drivers use their lights to communicate with other drivers. If an approaching truck flashes you, it usually means danger ahead.

While you are overtaking a truck, a blink in the truck's lights means that you can safely pull in since there is a safe distance between you and the truck. Without the blink, you would have to judge the pulling in clearance yourself, which may be difficult sometimes, especially with very long, articulated trucks.

Flashing any kind of vehicle lights is prohibited in many countries.