Cyclists made the most of the new $750 million Perth to Bunbury Highway by taking part in a bike ride this morning.
Official proceedings kicked off at 10am with speeches, the laying of a plaque and the cutting of the ribbon at the Pinjarra Road interchange.
The highway project, one of the biggest undertaken in WA transport history, consists of the extension to Kwinana Freeway plus 38.5km of dual carriageway, called the Forrest Highway, which extends to the start of the Old Coast Road dual carriageway, just south of Lake Clifton.
People wanting to drive down the Perth to Bunbury Highway may have to wait until this evening before police allow access to the road.
Southern Gateway Alliance spokeswoman Tammy Mitchell said police would close the highway after the opening ceremony finished at noon and would open it in stages after that.
Ms Mitchell expected cars would be able to drive on the highway by late afternoon or early evening.
"It will definitely be open by nightfall because we don't want people to be unsafe when they first drive on it," she said.
Transport Minister Simon O'Brien said the opening signified a new era for WA's road network.
"We should never forget, nor underestimate the contribution of the over 3000 workers who toiled for a combined 3.8 million man hours to bring this project to fruition," he said.
Ms MacTiernan said it was a magnificent project.
"There's a lot of work been done by so many people on this for 10 years and I think we should all get out there and say it's a great West Australian project," she said.
Investigators are still on site after four people died on the Thunder River Rapids Ride.