Statistical anomalies show problems with Chicago’s red light cameras

There has been a lot of fallout from the Chicago Tribune‘s report on problems with Chicago’s red light cameras. And the smoking gun was the improbable spikes in tickets handed out on single days or in short stretches:

From April 29 to June 19, 2011, one of the two cameras at Wague’s West Pullman intersection tagged drivers for 1,717 red light violations. That was more violations in 52 days than the camera captured in the previous year and a half…

On the Near West Side, the corner of North Ashland Avenue and West Madison Street generated 949 tickets in a 17-day period beginning June 23, 2013. That is a rate of about 56 tickets per day. In the previous two years, that camera on Ashland averaged 1.3 tickets per day…

City officials insisted the city has not changed its enforcement practices. They also said they have no records indicating camera malfunctions or adjustments that would have affected the volume of tickets.

The lack of records is significant, because Redflex was required to document any time the operation of a camera was disrupted for more than a day, as well as work “that will affect incident volume” — in other words, adjustments or repairs that could increase or decrease the number of violations.

In other words, graphs showing the number of tickets over time show big spikes. Here is one such graph from the intersection of Halsted and 119th Street:

As the article notes, there are a number of these big outliers in the data, outliers that would be difficult to miss if anyone was examining the data like they were supposed to. Given the regularities in traffic, you would expect fairly similar patterns over time but graphs like this suggest something else at work. Outside of someone directly testifying to underhanded activities, it is difficult to imagine more damaging evidence than graphs like these.

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