If you are driving behind a truck or a trailer truck, particularly on a narrow, winding or sloping road, you should watch the light signals given. When the truck is close to the left side of the road and the left turn signal is on, that means you can overtake. The right turn signal means you must not overtake as there may be another vehicle coming from the opposite direction. Alternating left and right turn signals mean you must not overtake as there is some obstacle or an accident ahead. With those warning signals from the truck in front, you should slow down your car and not try to overtake it. You may give the same signal to warn the vehicles behind you.
Giving such signals can be done both at night and during daytime.