Without quite knowing what the 'black holes' are, my guess would be for positioning of toll road transponder units.
A lot of windscreens, especially European manufactured vehicles, have heating elements built into the windscreen glass and therefore transmitters/receivers can't pick up signals from overhead road toll transponders.
I have a Range Rover and the transponder has to be situated on a particular part of the windscreen so that it can transmit its signal.My Fiat Ducato has to have the transponder in a certain spot for it to operate effectivley as well.
Perhaps that is what you are referring to.
If not, maybe somebody else knows what it's about.