It is a kind of informal language among road users . There are various types of these signals. However, they are not all officially defined and can differ from country to country. Therefore, if you see such a signal, you must always assess the situation. After all, a flash of light can mean something else at other times.
It is not official permission
Although many truck drivers understand these signals, they are not official traffic rules. In the Netherlands, according to traffic regulations, flashing and audible signals primarily serve a function in the event of imminent danger. Therefore, legally, a brief flash of light does not mean you are granted permission to change lanes. Nor does it give you the right of way. At most, the driver behind you can indicate that he sees sufficient space.
That difference is important. Therefore, always keep looking yourself before moving to the right. Check your mirrors and maintain sufficient distance from the truck . Also, do not drive directly in front of the truck. Heavy vehicles require much more distance to come to a stop. Extra space between your car and the truck is therefore always wise.
Not every flash of light means the same thing.
If you see a truck flashing behind you, pay close attention to the moment it happens. Does the flash occur exactly after you have overtaken the truck? Then there is a good chance the driver is trying to help you. If the signal occurs at a different time, something else might be going on. Perhaps the driver wants to warn of danger. It could also mean that another traffic situation requires his attention.
