Speed cameras monitor traffic in front of Wootton High School in Rockville. (Jahi Chikwendiu/THE WASHINGTON POST)

AUTOMATED SPEED cameras enforce the law, cost taxpayers nothing and make streets safer for everyone. So it’s hard to understand why they have been so controversial, with more than a dozen states going so far as to restrict or outlaw their use. An authoritative new study that clearly establishes that cameras save lives hopefully will jump-start more jurisdictions into putting cameras on their streets.

A report presented this week to the Governors Highway Safety Association projected that more than 21,000 deaths or serious injuries could be prevented with nationwide use of speed cameras. Researchers for the Insurance Institute for Highway Safety examined a program put in place by Montgomery County in 2007 and compared its experience with that of Fairfax County, where speed cameras are not used. They found a 59 percent decrease in the likelihood of a motorist in Montgomery County exceeding the speed limit by 10 mph or more. And because speed is a factor in more than 50 percent of fatal crash reports, getting drivers to slow down translates into saved lives and fewer injuries.

Cameras are an effective and efficient means of changing behavior. Fewer people will speed if there is a likelihood of being ticketed. Exclusively using police officers for traffic enforcement is neither realistic nor desirable: not realistic, because of other demands on their time; not desirable, because cameras enforce the law without the biases that can accompany traffic stops.

It’s important, of course, that the programs be well-managed with accurate cameras that are placed with safety, not revenue, as the goal. Montgomery County has a good model with its use of speed camera corridors that force drivers to watch their speed for the length of a road rather than one fixed location. Successful programs are also in place in the District, Prince George’s County and in countries such as France, which has seen dramatic progress in reducing traffic fatalities. Their experience should serve as a model. John Townsend, a spokesman for AAA Mid-Atlantic, told us the group hopes to use the report in its efforts to promote camera programs that are focused on safety and properly managed by law enforcement agencies.

Clearly no one welcomes getting a ticket, but drivers who do so should stop complaining, slow down and be thankful for a technology that has proved to save lives.