The first attack involved a newspaper box, which someone on an overpass hurled at Lois Bowes's photo radar van below. The next time it was a brick. This week it was a chunk of concrete.

Attacks against the Edmonton Police Service's mobile photo radar units have become so prevalent — 13 in the past 13 months —— that the force is looking at fortifying the vehicles to protect their operators.

"The vans are easy targets, and the potential for having somebody harmed inside the van is very high," spokesman Wes Bellmore said.

Several cities including Calgary and Winnipeg use the vans, which photograph speeding cars, whose owners then receive traffic tickets.

And Ontario Premier Dalton McGuinty has mused about reinstating photo radar to help battle the deficit.

And where there is traffic enforcement there is rage. British police estimate that 400 mounted speed cameras have been vandalized over the past two years. In Israel, drivers have shot at the devices.

Despite the onslaught, Edmonton's force remains committed to photo radar, which it says has saved lives.

"The problem with photo radar is it's a love-hate thing, and the people who hate it are very vocal and very passionate about it, and the bulk of the drivers just drive along, happy that it's reducing speed," Mr. Bellmore said.

In Edmonton on Monday night, Special Constable Bowes was parked off a thoroughfare when a flying piece of concrete smashed her unmarked van's rear window. She escaped unscathed, but she and the force's other 18 operators worry someone will be seriously injured.

"We feel it's going to happen one of these days. ..... You look around all the time, but you can't look everywhere. It keeps the heart pumping," she told the Edmonton Sun shortly after the incident.

Most of the city's recent cases — one of the incidents involving Special Constable Bowes happened in 2001 — have involved objects thrown through the vans' windows. Officers believe at least some of the cases are connected, although few perpetrators have been arrested or charged.

"In general, there's not much hope for a lot of these incidents," Mr. Bellmore said.

But in one brazen — and foolish — instance last week, a man whose motorcycle tripped the photo radar camera came back to exact his revenge by spitting on the van's driver-side window as the operator sat inside. The man left, but came back again and kicked the van, damaging an antenna and causing about $200 damage. He was charged with mischief and impaired driving.

"Clearly, there's an utter disrespect for the law, especially when you attack the law as it's being enforced," said Don Szarko, a spokesman for the Alberta Motor Association.

The profile of those responsible for the attacks, Mr. Bellmore said, is similar to road-rage motorists and then there is the aggravating factor of a financial penalty.

"We believe we're dealing with the same type of person," he said. "It's obviously an impulsive, immature type of person doing this. We're not dealing with someone who's fully in charge of their emotions."

Those responsible for the attacks have come to recognize the force's five minivans, which have a camera and flash unit mounted to the front bumper . The force makes no secret of their locations in order to keep drivers on their guard.

While officers have not yet determined what they will do to increase operators' safety, the measures under consideration include installing shatter-resistant glass in the vans or even using vehicles without side and rear windows.

Most of the revenue from Edmonton's photo radar enforcement is directed to the police operating budget.