There's got to be a better way to confirm the existence or non-existence of such must-avoid intersections.
Live traffic data I suppose. Traffic signals will calculate degree of saturation from dwell times on induction loop vehicle detectors. In most systems that data is passed up the chain to the software which does strategic traffic management. I have been out of the area for a while but I assume the live data is extracted at this point and aggregated into these live traffic data sets.
This is done in Japan actually - a system called VICS feeds live traffic data back into navigation systems. Granted they have the advantage of standardization and a much smaller traffic network, but I don't see why it couldn't work in the states.