Cameras at stop lights in the U.S. are becoming more common, and cities are relying more and more on the revenue those cameras can provide through tickets for running red lights. However, a new study by researchers at the University of Tennessee has shown that tuning red lights to increase revenue can actually increase the risk of traffic accidents.

The study, published recently in the journal Transport Policy, found that the better red light systems are at increasing safety, the less profitable they are for cities and the companies that provide such systems. Methods for revenue generation, such as decreased yellow light times or increases in the speed limit, were shown to increase the incidence of drivers running red lights.

“One of the major challenges with implementing red light camera policy is the conflict of matching incentives of tangible revenue for industry and the municipality contrasted with external cost savings such as safety and congestion the value of which is not easily captured,” said Chris Cherry, a co-author of the study. “We hope the public sector and the public use our research to reflect on the motivations for changing signal operations.”

Shortening stoplight cycle length and increasing the time of all-red lights were also shown to increase red light running. Shortening the length of yellow lights while also increasing speed limits was shown to increase the chance of crashes.

The study’s authors assert that traffic engineers will have to decide on their own how to tune red light systems. They suggest that a combination of methods could be used to produce revenue without causing traffic or compromising safety.

“Traffic engineers are facing an ethical dilemma of balancing revenue generation to sustain their red light camera programs with their traffic safety and efficiency goals,” said Lee Han, a co-author of the study. “This is a new conundrum for them.”