Allegations that some windscreen washers at Perth intersections are being intimidating and aggressive and committing damage, assaults and robberies have sparked police crackdowns and a warning not to encourage their illegal ventures.

Belmont MLA Cassie Rowe said it was a constant problem in her electorate but she was seeing another spike in complaints after increased trouble earlier in the year.

“I was shown video footage recently of young women at the intersection of Great Eastern Highway and Graham Farmer Freeway and you could see they were agitated and aggressive towards the driver of a ute who appeared to be declining to have his windscreen washed,” she recounted. “They were screaming and kicking the car and then they threw something into the car window and were leaning in to try to hit driver.

“I’ve been approached by so many locals in so many different suburbs... people are sick of the aggression and harassment.”

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Less than a fortnight ago, Belmont police officers allegedly interrupted three teenage windscreen washers trying to rob a man of his bicycle at Great Eastern Highway.

One grabbed the handlebars and demanded the bicycle, the second punched him and a third told him “give me your bike or I’ll stab you”, police claim.

Officers caught two 18-year-olds, who have since been charged with attempted aggravated robbery, and are looking for a third young man.

Belmont Police Sen Sgt Frank Anderson said cleaning windscreens at traffic lights was illegal and attracted a $50 fine and those doing it put themselves at risk of being struck by vehicles.

Police regularly patrol the known hotspots of Great Eastern Highway and the Graham Farmer Freeway, the traffic lights at Tonkin and Great Eastern highways and the intersection of Belgravia Street and Great Eastern Highway.

Sen Sgt Anderson said officers investigated all criminal allegations and were trying to use bail conditions and court-ordered prohibited behaviour orders to prevent offenders continuing their roadside trade.

Across the river, the intersections of Roberts Road and Thomas Street – near Perth Modern School – and the exit of the Graham Farmer Freeway at Cambridge and Thomas streets are also on the police watch list.

Wembley Sen Sgt Peter Gilmour said a core group of windscreen washers caused trouble, not only for motorists but school students and other pedestrians.

He said police would penalise and move them on but they returned because some motorists did pay them.

Sen Sgt Gilmour said it was also an offence to pay for the illegal service and while police had not been penalising motorists, he urged them not to have their windscreens cleaned to discourage the venture and improve safety.

“Some people feel sorry for them but in reality both the windscreen washers who are soliciting to get money from drivers are committing offences and also the drivers paying these people are committing an offence,” he explained.

Sen Sgt Gilmour also urged motorists not to engage with the youths beyond saying ‘no thanks’ to avoid confrontations.

Ms Rowe said police were doing a fantastic job to try to stamp out bad behaviour.

But she backed their calls for motorists to ignore windscreen washers, saying there was little incentive for them to stop while it was financially lucrative.

“I am sympathetic that some of them are young and just trying to make money but it’s not safe for them (on the roads),” she said. “It’s really intimidating for local residents, they’re getting harassed, they feel they’re being targeted in a really aggressive way if they decline to have their window washed. “It seems to have really increased and people are sick of it.”

Police encourage people to report crimes and say dash-cam footage of incidents is welcome.