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Congestion at traffic lights and emergencies

I have noted before the problems of setting the "stop"/"go" sequences at traffic lights.  When the media quote the average time that a resident of X (X may be a city or country) spends in queues each year, invariably traffic queues feature as a source of delay.

So, the other day, I was stuck in a queue at one of Exeter's crossroads.  It is one that I knew from my daily commute to the university, so I had measured its performance.  Four roads are controlled by the lights.  Let us call them North, South, East and West.  The normal light sequence is:
A) Green for East and West, red otherwise
B) Green for South, red otherwise
C) Green for East and West, red otherwise
D) Green for North, red otherwise
E) All red to allow pedestrians to cross

Any of A)-D) may be omitted if the detectors show no traffic waiting and E) may be omitted if no pedestrians have pressed their signal.  A)-D) are time-capped at 30 seconds, and are also capped if there is no detectable movement o…

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