Red light cameras did nothing to improve safety during the years they were used in Houston, Texas. That was the conclusion of a recently published study by Case Western Reserve University economist Justin Gallagher and Paul Fisher, a graduate student at the University of Arizona. Their paper, published by the Social Science Research Network, covered twelve years' worth of accident data at photo enforced intersections in Houston and Dallas.

Houston provided a perfect test scenario for the analysis since voters outlawed the use of automated ticketing machines in 2010, but the devices are still in use in Dallas, where the researchers analyzed the ongoing effect of camera use at control intersections.

"If electronic monitoring in Houston is successful at improving traffic safety, then we expect that the removal of the cameras would lead to an increase in the number of total accidents and injury accidents at camera intersections, relative to control intersections not subject to the referendum," Gallagher and Fisher wrote.

Houston began automated ticketing in 2006, with its private contractor mailing out over 800,000 tickets worth $44 million. The city insisted the use of cameras reduced accidents, but the study notes that red light cameras were installed at locations with abnormally high accident numbers in the year before installation and they naturally fell back to average levels (a statistical phenomenon known as regression to the mean). This effect was accounted for by the unexpected voter-led removal of the cameras.

The study noted that most red light camera tickets were issued within the first second of the traffic light changing from yellow to red, when opposing traffic still had a red light. Ticketing this behavior would have little effect on the sort of red light running that causes angle accidents. Encouraging unexpected braking maneuvers on a yellow light, on the other hand, could increase rear end accidents.

"Drivers may simply miscalculate," Gallagher and Fisher explained. "The decision to stop or continue is a split-second decision. For example, knowledge of the cameras (perhaps cued by the posted signs), could lead some drivers' first impulse be to stop even when it would be safer to continue through the intersection."

As a result, the study found non-angle accidents increased between 18 and 28 percent after red light cameras were installed. Once cameras were removed, there were slightly fewer total accidents -- despite an increase in angle collisions.

"Overall, we find no evidence that cameras reduce the total number of accidents," Gallagher and Fisher found. "We estimate a statistically insignificant reduction in total accidents (-3 percent) after the cameras are turned off."

Non-angle accidents are twice as common as angle collisions, and those non-angle accidents were not minor fender benders. Injuries decreased once Houstonians kicked out the photo enforcement vendor.

"There is no evidence that the electronic monitoring led to fewer accident-related injuries," Gallagher and Fisher found. "Estimates from the Houston sample suggest that the camera program may have increased injuries. The point estimates are all negative after the program ends, and are marginally statistically significant for a reduction in injury accidents."

The study notes that injury accidents are so rare that it is hard to draw firm conclusions from the numbers.

A copy of the study is available in a 1.3mb PDF file at the source link below.