In 2016, a kilometer charge for trucks over 3.5 tons was introduced in Belgium. That means that those vehicles must have an activated On Board Unit on board on all Belgian roads. Since the measure was introduced, almost all drivers have been complying with the regulations. Meanwhile, the system and its controls have evolved and can better target some behaviors.
We see that a number of things could need clarification.
This is the way to avoid fines as the controls are frequent, around the clock and on all roads.
Have a pleasant journey!