City traffic engineers chalked up the swings to lanes being closed and reopened during bridge construction nearby.

As the camera system has expanded in Maryland, most data shows crashes have gone down, although a closer examination shows mixed results.

Baltimore reported 19,529 traffic accidents last year, a decline of about 270 from two years earlier. The number of fatal crashes dropped by almost half over that time, to 22. Accidents in Baltimore County fell from 13,254 to 12,433, and fatal crashes dropped from 79 to 70. Statewide, accidents and fatal crashes also decreased.

Yet some categories of crashes, such as those involving pedestrians, rose in both the city and Baltimore County. And for the first half of 2012, as speed camera use in Maryland hit a new peak, the number of fatal crashes statewide was 17 percent higher than the year before.

While crash statistics fluctuate, over time "you can be pretty confident that reductions are going to happen," said Nancy K. Fitton, a Cochrane Center coordinator at the Johns Hopkins Bloomberg School of Public Health.

However, the Cochrane Review's authors said "higher quality" studies are needed, particularly in the United States, where camera data hasn't been examined as closely as it has in Europe and Australia. Other experts share that assessment.

"We need that definitive study," said Roy E. Lucke, who directs the Highway and Transportation Safety Programs at Northwestern University's Center for Public Safety. "It hasn't been written yet — on the true effectiveness of speed cameras on reducing speed in the areas where they're installed."