Find root blockers

The blocked-by graph is a useful tool for finding immediate sources of blocks. Its uses would be expanded if there were a root-blocker dictionary that were able to identify the red light holding up a line of cars as the root cause for every car in that line. There are some tricky situations that arise when considering gridlock (cycles in the blocked-by graph), but I'd think those can be can be treated as exceptions.

For some context, in the real world, when someone says "I've been waiting at this light for 10 minutes," it doesn't necessarily mean that they have been at the front of the queue for that long. Also, if someone is second in line at a red light, they are much more likely to say "I'm waiting at a red light" than "I'm waiting for this car in front of me."