Six U.S. cities have been found guilty of shortening the amber cycles below what is allowed by law on intersections equipped with cameras meant to catch red-light runners. The local governments in question have ignored the safety benefit of increasing the yellow light time and decided to install red-light cameras, shorten the yellow light duration, and collect the profits instead.
Indeed, the incentives involved for the government are very perverse. If you reduce the yellow light duration, you'll get a bit more revenue. If you lengthen the yellow light duration, you'll reduce accidents and save lives. So public officials choose to....reduce yellow light duration?
[insert comment here about the monopoly power of government]
Shortening the yellow light below the minimum time is clearly wrong - i.e., illegal and unethical - but what about the more general disparity in yellow light duration? Some yellow lights last 4 seconds, others 5 seconds, others 6 seconds, etc. How can drivers make the stop/go decision with confidence when there is such a large disparity in yellow light duration?
Even if they don't shorten yellow lights below the minimum, drivers are unlikely to know until after they've been caught just how long a yellow light might be. That also seems problematic from a legal and ethical standpoint. Imagine a law against loitering under which you could be charged (without prior warning) for remaining in one place for too long...but the duration was not posted and you were not informed until after you had been ticketed/arrested. That would be clearly problematic, but how is it very different from the yellow light disparity?
The reason for disparities in yellow light times is obvious (speed, intersection size, etc), but the result is a law about which drivers are forced to make split-second guesses. And cities have an incentive to stack the deck against them.