The warning triangle is an item required to pass the car inspection. The triangle can be found on the back of the Kekmet; just right click it to detach it from the tractor. It cannot be attached to the Satsuma; just keep it somewhere inside the car during the inspection.

Placing the warning triangle behind a vehicle when on the side of the road will not affect the AI; it will neither make them slow down or move to the side, so after the inspection the triangle is futile.