Sioux City has automatic red-light running detection cameras. These are doing their job. In the Sioux City Journal by Molly Montag article on the topic, the facts are all clear, unfortunately the headline takes the small negative instead of the large positive as the lede: “Sioux City data: Rear-end crashes increased at 5 red-light intersections”

In Sioux City’s case, new police data obtained by the Journal show a 40 percent decrease in crashes from motorists running red lights at intersections with cameras and a 15 percent reduction in all crashes. Iowa Department of Transportation data also show a decline in accidents involving red-light running. “In general, this is a positive result, as rear-end crashes (though not desirable) are not as severe (or dangerous) as right-angle crashes that might otherwise occur,” David Levinson, professor of transportation engineering at the University of Minnesota, said in an email.

University of Missouri-St. Louis transportation studies professor Ray Mundy said rear-end crashes typically increase after systems are installed and drivers slam on the brakes when they see the camera flash. Such accidents usually decrease over time as people get used to driving through camera-controlled intersections. He echoed Levinson in saying more slower-speed rear-end crashes are a tradeoff for reducing higher-speed, T-bone crashes that happen when drivers run red lights.

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The University of Minnesota’s Levinson agreed the decrease in red-light crashes showed the systems improved safety.

“That is the important takeaway,” he wrote.

I looked at the data. While in my analysis the total number of crashes did not change, it is clear that automated traffic enforcement reduced the number of “ran traffic signal-involved” crashes and increased the number of “rear-end-involved” crashes. The differences before and after installation by crash-type are statistically significant and meaningful in both cases. This is consistent with general results nationally about the effects of automated traffic enforcement. (And what you would expect if there were more sharp braking at intersections as people strive to avoid fines).

Cost to users is a transfer to the city (and should otherwise reduce some other taxes the city is collecting), and though there is some cost to administering the system, that is outweighed in general by the safety benefit.