Fixed speed cameras on the freeways and Roe Highway are still catching thousands of speeding drivers, racking up more than 17,400 infringements in six months.

An analysis shows the cameras have been effective in reducing speeding at the fixed locations but reveals they are operating only about 15 per cent of the time, raising concerns the deterrent effect is waning.

Monash University professor Max Cameron said using the cameras too infrequently could give speeding motorists the impression they could get away with their dangerous behaviour.

He said it was surprising so many road users were still speeding past cameras, despite the sites being well known.

WA Police statistics from the past 17 months show the four fixed cameras on Mitchell Freeway, Kwinana Freeway and Roe Highway are on average used only for 108 hours, or 4 1/2 days a month, with cameras not operating at some sites for five months at a time.

Cameras inside the fixed housings at the four locations are also used by mobile speed camera operators and are not always available for use.

At the Mitchell Freeway, near the Erindale Road site, the camera was in use only 9 per cent of the time and was absent for 10 months between January last year and May this year.

The Kwinana Freeway and Roe Highway site cameras were in use for 13 and 14 per cent of the time respectively.

The State's first fixed speed camera on Mitchell Freeway, near Karrinyup Road, had the highest average operating hours, at 23 per cent.

Acting director of State traffic operations Tony O'Donoghue said it was not necessary to have the cameras operating all the time and they had achieved a big reduction in motorists speeding in those areas.

"The effect we're trying to get is to encourage motorists (to stop speeding), so it's an any time, anywhere situation where the motorist is not to know which of the devices are operating at any one time," he said.