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The last we heard from Houston about its red light cameras, in 2011, the city had not only shut them off but outlawed them—a culmination of months of rage against the machines. Over at Streetsblog, Angie Schmitt reveals the safety legacy of that decision, and it isn’t pretty. Police numbers show an astonishing 116 percent increase in total crashes and an 84 percent rise in major crashes at former camera locations from 2010-2014, compared with the four prior years when the cameras were up and running.

The Houston PD’s before-and-after comparison isn’t perfect, lacking context about broader traffic safety trends across the city. But the basic gist echoes loads of other evidence pointing to the safety benefits of red light cameras.

Generally speaking, red light cameras do an excellent job of reducing “red-light related” crashes—the highly dangerous T-bone collisions that occur when a driver blows a red once other traffic has entered an intersection.

Drivers are also disturbingly quick to regress to the bad habit of running reds when the cameras go dark. The risk of running a red in Southeast Virginia, for instance, rose three times in the immediate aftermath of a camera program. And while cameras have a mixed record on total crashes, some jurisdictions do find sizable declines.

That’s all great news for city safety. But two things perpetually keep the case for red light cameras something well short of airtight.

Increase in rear-end collisions

Drivers hoping to avoid getting nabbed by the camera will often speed up through a yellow, only to knock into a car that’s stopped in traffic on the other side. Recent academic studies in Charlotte and Los Angeles have found post-camera increases in rear-end collisions. The Chicago Tribune recently conducted its own study of the city’s camera program, and while it did find a 15 percent decline in right-angle crashes, it found a 22 percent rise in rear-end collisions, too.

The Wall Street Journal recently wondered if camera companies like Redflex have a future. Yousuf Fahimuddin / Flickr

The Tribune’s results track with the most official study of red light camera programs to date: a 2005 investigation of 132 camera sites in the U.S. by the Federal Highway Administration. In keeping with the general balance of safety benefits, FHWA did find a 25 percent dip in right-angle crashes alongside a 15 percent rise in rear-end crashes. The agency concluded that camera programs had a “modest aggregate crash-cost benefit” after considering the economic impact of both types of crashes, but the rise in rear-end collisions makes the safety argument muddy.

Rampant corruption

Time and again red light camera companies, as well as the cities that contract with them, have been suspected of tweaking yellow lights in such a way as to increase ticket revenue. As Yonah Freemark wrote here in 2011, some contracts actually “require each camera to record a certain number of red light-runners every year.” Chicago’s recent camera scandal, exposed by the Chicago Tribune, resulted in thousands of improper tickets issued to city drivers:

The experts all said the available evidence leads them to only two possible explanations—that ticket procedures were quietly broadened to catch more violators, or that malfunctions led the system to wrongly tag lawful drivers. In either case, they said, fail-safes that should have guarded against such anomalies didn't do their job.

Together the spike in rear-end crashes and the official malfeasance has reinforced a resistance to red light cameras that might otherwise have rested on little more than gripes over the occasional fine.

That might explain why red light camera programs are on the decline. Down from a reported high of 540 communities with cameras in 2012, only 443 have them today, according to the Insurance Institute for Highway Safety. The Wall Street Journal recently wondered if red light cameras have a future at all.

There might be some middle ground to salvage yet. Extending yellow lights, which has been shown to decrease rear-end collisions, might help. With new leverage over the reeling camera industry, cities can force more responsible contracts that prioritize safety over revenue. When Houston’s red lights went dark back in 2011, we predicted that some other cities would follow suit. But given the clear public health benefits these cameras can provide when used correctly, there’s still a mandate out there for others to lead.