The speed bumps were installed last year after complaints about speeding in the area.
A traffic survey confirmed there was a high amount of speeding on Bullarra Road and Woolowra Road.
Greenmount resident and St John Ambulance paramedic Craig Van Der Laan contacted the Hills Gazette about the speed bumps late last year.
Mr Van Der Laan said he was angry at the decision by the Shire to install the speed bumps on the roads, which he said was redirecting traffic past a local school.
He said cars were driving over the verge and footpath to get around the speed bumps.
Shire of Mundaring chief executive Jonathan Throssell said the Shire recently placed bollards alongside the speed bumps in Bullarra Road to stop motorists from driving on the verges and footpath.
‘This is illegal and the WA Police have booked motorists for this practice,’ Mr Throssell said.
‘The speed cushions are installed to slow traffic. Speed cushions have proven to be successful in slowing traffic speeds.
‘Woolowra Road was not experiencing the same difficulties and has not had any further treatment installed.’