Speed cameras are getting smaller, smarter and more mobile as local governments increasingly turn to them to nab speeding drivers.

Prince George's County has quadrupled its fleet of speed cameras over the last year with the purchase of nearly five dozen mobile cameras, which are battery-operated and can be easily relocated, unlike the more commonly used fixed-pole cameras, which require underground wiring. The county gave out more than 349,000 citations in the first nine months of the program, which began in September.

Government traffic officials say the mobile cameras -- which are less visible from the road and are housed in mailbox-like cabinets -- keep drivers guessing, so they are more effective in slowing traffic.

"If people get used to where our cameras are, they slow down for that particular spot and then they resume speeding above the limit down the road," said Robert McCullagh, deputy director of the Montgomery County Police Department's Traffic Division.

Montgomery recently doubled its inventory of mobile cameras to 20 and adopted a "corridor approach" to catch speeders with the moveable ticketing machines, he said.

Under the new approach, law enforcement officials rotate mobile cameras along stretches of road where speeding is common. The Traffic Division is planning to advertise the speed zones but it won't provide details on the cameras' exact locations, McCullagh said.

No law requires the department to post any signs notifying drivers of the cameras unless they are in a school zone, he said.

Opponents say the mobile cameras are a ploy to generate more revenue from unsuspecting drivers.

"The older cameras are tall and very visible to drivers from a long way off," said Ron Ely, editor of the website StopBigBrotherMd.org, which tracks speed camera policy in Maryland. "These newer [mobile] cameras are lower to the ground and hidden between bushes and signs. Not only can they move these cameras, but they are also less visible from the road."

Chevy Chase Village, which has speed cameras along a six-lane stretch of Connecticut Avenue between Bradley Lane and the D.C. border where the speed limit is 30 mph, has put in green mobile cameras near shrubs.

Each traffic citation issued by a speed camera in Maryland costs $40, adding up to more than $16 million collectively in net revenues for Montgomery and Prince George's counties in fiscal 2012.

Meanwhile, D.C.'s camera program, which has rapidly expanded since 2008 with the addition of dozens of mobile cameras, netted $55 million in revenue in fiscal 2011, according to AAA Mid-Atlantic. Tickets in D.C. can cost drivers up to $250, depending on how many miles over the speed limit they are caught driving.

Virginia doesn't allow speed cameras.

The money is a boon for cash-strapped governments, but revenues from the cameras have been falling in recent years, especially for older, fixed-pole camera sites that drivers have adjusted to.

Rockville swapped two of its 10 fixed-pole cameras for mobile cameras in 2010 when the citations coming from those locations dropped precipitously, with one location seeing an 80 percent drop in the rate of speeding violations. Fewer citations meant less revenue: The city's camera earnings in fiscal 2010 were $2 million, down from $4.1 million two years earlier. The city now has seven mobile cameras and two vehicle-mounted cameras, in addition to its eight fixed-pole cameras.

"Using [mobile cameras] on a rotating basis at various locations along the same section of roadway and on nearby roadways, we have almost created a series of enforcement zones around the city," said Michael England, director of the Rockville City Police Department's Special Operations Bureau. "As a result, motorists are changing their driving habits and slowing down."

hpeterson@washingtonexaminer.com