There are 3 different matrix displays in game. All three are activated by selecting the vehicle and clicking the LED button. Most buttons have several functions stored in them, which can be switched through by clicking several times.
They can be turned off by sending the vehicle home or by using the leftmost button.
Here are a few example vehicles with their respective matrix and a few examples of the functions.
City cleaning and some vehicles of the fire department for example the Lüdenscheider 14-TLF3000-01
Arrows to the left/right and cross
patrol car:
Arrows to the left/right + accident indication
Full blockage display
Armored personnel carriers/city cleaning/Linzig 01-ELW1-01:
Speed limits
Traffic guidance displays
Danger signs