There’s nothing wrong with city or state government taking measures to keep our roadways safe. But the measures they take should be effective. If they are punitive, the measures should give motorists due process; and there should be minimal potential for abuse. Traffic cameras fail on all three counts.

A recent BBC study of mobile speed cameras revealed significant accuracy problems. One researcher was able to clock a stationary wall at 58 mph. The Australian government has begun paying $26 million back to motorists who were issued tickets by faulty cameras. A Canadian town recently recalled 6,800 tickets issued by cameras. The Washington Times has reported several incidents in which D.C. motorists were ticketed for cars they no longer own or drive, or that are inoperable.

Clearly, speed cameras err. But motorists issued tickets might be surprised to learn that they’re generally considered guilty until proved innocent. It’s up to a car’s owner to prove he wasn’t driving when the ticket was issued, that the camera misread his plates, or that the camera itself is faulty. In many cases, the private companies who run the cameras get a cut of each ticket issued, and appeals of tickets are settled by the company itself, not a judge or traffic court. Camera manufacturers have also been known to train cops on how to testify against appeals, and in some cases have paid officials to advocate the cameras to other cities. That’s an unsettling kind of justice.

Cameras are ripe for abuse, too. One city in Florida now uses traffic cameras to snap the plates of every motorist entering the city, and checks them against various law enforcement databases. In Southern California, a photo taken by a red-light camera was used to prove adultery in a divorce case. San Diego shut down all of its cameras after a judge ruled that the company in charge was tampering with the machines to increase ticket output.

But the most troubling thing about traffic cameras is the way city governments grow dependent on the revenue they generate. Bethesda, Md., was caught shortening a yellow light at the city’s most lucrative red-light camera, in an effort to squeeze more cash out of its motorists. When tickets dropped off from existing speed cameras in Washington, the City Council simply installed more, and raised the fines. Sacramento now charges motorists $351 for a single red-light violation.

Traffic enforcement should be primarily about safety. Too often with cameras, it quickly devolves into generating revenue for local governments and the companies that operate them.