There are also privacy concerns—the fear, as one editorial in Canada’s Globe and Mail recently expressed it, that cameras “could turn every officer into a mobile, closed-circuit camera, hooked up to a database tracking and recording people’s movements across the city.” People could refrain from reporting incidents to the police because they don’t want to appear on camera. Ariel is less worried about increased surveillance—”I don’t think there’s a single street in London that isn’t covered by CCTV [closed-circuit television],” he points out—than about what data is stored and how that evidence is used. Those are questions that each country’s legal system will have to sort out.

There is another potential cost to widespread adoption of body-worn cameras: The credibility of police testimony against defendants could be discounted in the absence of footage to corroborate the officer’s version of events. It’s the legal equivalent of “pics or it didn’t happen.”

Ariel gave the example of a traffic cop who spots an infraction out of the corner of his eye, and then can’t produce body-worn video of the episode. In that scenario, a judge could toss out the case for lack of evidence. “That has serious implications for policing and the status of officers’ testimony and statements in the court of law,” he said.