The Department of Transport has approved the application despite the City of Joondalup deciding in January to not support the application.
They were proposed to be primarily used for commercial advertising, but would also be used to display community safety messages.
Main Roads WA spokesman Dean Roberts said stringent selection criteria were applied to ensure the signs would not be placed at critical driver decision-making points.
�Main Roads WA will ultimately be responsible for managing the operation of the billboards and reviewing and approving all advertising content,� he said.
�Revenue from the signage will be reinvested back into traffic and safety improvements and congestion management.
�Road authorities in New South Wales and Queensland have similar commercial advertising activities.�
One 12.66m by 3.35m sign is proposed for the western verge of the freeway in Duncraig, 440m south of the Hepburn Avenue off-ramp, and the other is proposed for the eastern verge in Greenwood, 900m north of the Warwick Road exit.
They are expected to be installed by the end of June.