It's the third traffic fatality in two days, following Tuesday's three vehicle collision in Mt. Hope on Highway 6 that killed two men.
The woman was ejected from a red Suzuki after crossing over between the westbound to eastbound lanes near Waterdown Rd. at approximately 2 a.m. Wednesday.
OPP Sgt. Kerry Schmidt said the woman "lost control went through the centre dip. The (lone) female driver was ejected from the vehicle she landed on the eastbound lanes of the 403 where she subsequently struck by another vehicle."
He said it's unclear if she was wearing a seat belt at the time, or if alcohol or any other substances played a factor.
One of the westbound lanes was closed in the early hours of Wednesday for an investigation, but all lanes on the eastbound side were closed until shortly roughly 7:45 a.m., snarling the morning traffic from the Hamilton and Niagara areas towards Toronto. Before the lanes were reopened, traffic was parked for 8 kilometres behind Waterdown Rd., near the Aldershot GO Station.