The three lanes from the freeway under Roe Highway have been extended and vehicles entering from Roe Highway now have a dedicated lane.
More than 52,000 cars use the stretch daily, with that figure expected to climb to 75,000 vehicles by 2021.
The widening was stage one of a $77.5 million upgrade, with State Transport Minister Dean Nalder saying work to widen 3km of freeway between Armadale Road and Russell Road to three lanes should start next month.
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READ NOWSouth Perth contractor Brierty was awarded the contract earlier this month. Construction will finish mid-2016.
Cockburn Mayor Logan Howlett said the city was looking forward to the completion of the works up to Russell Road as soon as possible.