Red light cameras in Chicago were ticketing drivers with yellow light times under 3 seconds.
"It's showing 2.9, it records 2.9 on the data bar as you see on the violation," Transportation Commissioner Rebekah Scheinfeld said in a recent interview. "But that actual performance is probably 2.998 — or something like that — where the variation is in the hundredths or thousandths of a second, which is imperceptible."
"City officials said the yellow light times being rejected by judges as too short are in fact valid because they fall within an allowable variance that is caused by fluctuations in electrical power."
Yes, variations in electrical power, that's definitely it...
http://www.chicagotribune.com/news/ct-red-light-camera-yellow-timing-20141009-story.html
‘Entrapping drivers into running red lights’
Together, the yellow light and the all-red interval add up to what’s called a “change period.” That “change period” at the intersection where Gigov got his ticket equals the three-second yellow light, plus one or two seconds for the all-red interval -- a total of four or five seconds. Engineering practices would yield a nearly similar result: a 3.7 second yellow light, followed by 0.47 second all-red interval, totaling 4.17 seconds.
The difference is, Chicago shortens the yellow portion of the change interval, and lengthens the all-red portion.
“So from a safety standpoint, it’s probably OK, but the thing is they’re misallocating the times,” said Warren, “and so they’re basically entrapping drivers into running red lights.”
http://www.wbez.org/news/are-chicagos-shorter-yellow-lights-unsafe-or-just-unfair-110955
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