PHOENIX — So how does Gov. Janet Napolitano plan to generate $90 million to balance the state budget with photo radar?

This is one of the 20 new photo radar units being acquired by the state Department of Public Safety to deploy on roads around the state to slow down speeders — and, if Gov. Janet Napolitano gets her way, help balance the budget. The vans are outfitted with cameras (insert) which are not generally visible unless you open up the hatch. (Capitol Media Services file photos)

 Lots of motorists who like to drive fast — and ignore signs. An lots of radar units to catch them.

 It works like this.

 The Department of Public Safety plans to have 100 photo radar cameras. That includes both 20 mobile units and permanent cameras.

 Based on the experience of the photo radar cameras along the Loo 101 in Scottsdale, the governor’s office figures they will catch, on average, five motorists an hour, 24 hours a day, doing at least 11 miles per hour over the posted speed limit.

 OK, that gives you 43,800 violations a year from each camera — or more than 4.4 million drivers a year who can’t — or don’t read: The law requires signs to be posted when photo radar is used. In fact, the portable ones the DPS has for the mobile vans are about four feet tall and fluorescent orange and have to be located at least 100 yards ahead of the photo radar unit.

 Even assuming the cameras snap that many speeders, gubernatorial press aide Jeanine L’Ecuyer acknowledged that doesn’t translate to 43,800 citations. The figure, she said, is more like 24,090.

 There are a host of reasons, ranging from missed shots — the camera didn’t actually capture the vehicle clocked speeding — to those license plate covers which make it impossible to read the letters and numbers.

 Out of state license plates present their own enforcement problems.

 And L’Ecuyer said only about 55 percent of tickets issued actually would be paid. Leaving aside scofflaws, some people will choose to attend traffic school.

 That isn’t necessarily cheaper, as the “tuition’’ generally mirrors the fine. But completing the class erases the violatio from Motor Vehicle Division records, meaning no points on the license — and no surcharge on insurance.

 And vehicle owners who get citations in the mail can escape payment if the photo shows someone else was driving. Arizona la makes the driver responsible. And there is no requirement for registered owners to volunteer the identity of the actual driver.

 That leaves 13,250 paid tickets. Figuring the average fine a $157, that would put gross revenues at $208 million a year.

 But that’s before all the expenses of leasing the vehicles and installing and maintaining the cameras and radar units, a figur DPS pegs at about 40 percent. That brings the net take down to $124.8 million; Napolitano said she is using that $90 million figure because all the units will not be operating when the budget year begins July 1.

 There’s one other factor: While the decision to use photo radar belongs to the governor, she cannot unilaterally use the revenues to balance the budget: Without a change in law, the state gets zilch.

 Current law spells out that all fines collected by each city’s magistrate courts go to that city; justice court fines fatten up the county budget.

 Napolitano is banking on lawmakers agreeing to change that so that all revenues from photo radar tickets issued on state roads go to the state treasury. That leaves the cities and counties with the cash they get now — including money from tickets written by DPS officers who actually pull motorists over — but just cuts them out of the take from the new technology.