Two people who died when their ute plunged off a bridge in Keilor East, in Melbourne's north-west, may have been drag racing.

Key points: Police believe cars racing at speeds of 160 kph before crash

Police believe cars racing at speeds of 160 kph before crash Driver of silver sedan called to come forward

Driver of silver sedan called to come forward Traffic delayed after outbound lanes on Western Ring Road closed

Traffic delayed after outbound lanes on Western Ring Road closed Crash happened after illegal street party in Campbellfield

Police said two cars were speeding along the Western Ring Road at 1:30am when the driver of one of the vehicles lost control and drove off the side of the EJ Whitten Bridge, which is 55 metres at its highest point.

The ute, which police estimate was travelling at 160 kilometres per hour (kph), crashed into a one-metre-high barrier before landing in a deep gully and bursting into flames.

A witness told Macquarie Radio a fireball was sent up into the air when the car crashed.

"I was driving to work at 1:30am and I was doing 100 kph and these two cars came flying past me and gave me a fright," he said.

"It would have been 100 or 200 metres in front of me. I saw one of the cars lose control and go over the railing.

"It burst into flames, it was surreal. It was like watching a movie."

The crash at Keilor East caused heavy traffic delays for morning commuters. ( ABC News )

The crash caused heavy traffic delays for morning commuters with the southbound lanes of the Western Ring Road closed while police investigated.

One lane remains closed.

Inspector Bernie Rankin said the second vehicle, a 2003-2004 silver Ford sedan with a green P-plate, had earlier been seen at an illegal street party attended by hundreds of youths in Campbellfield.

He said the driver of the sedan was later seen baiting other vehicles to race before the crash.

"This vehicle was seen encouraging other vehicles to get involved in a drag race," Inspector Rankin said.

"This vehicle was seen entering the Western Ring Road, entering from Sydney Road, then it travelled south and at some stage has got into a drag race with the vehicle where our two deceased people have been located.

"This is a classic case of testosterone, adrenalin, young fellows and fast cars. It's just a dreadful mix and we've had very, very serious consequences."

The ute crashed into a one-metre barrier before it drove into a gully and burst into flames. ( Supplied: Channel Nine )

He said police were investigating why the ute driver had lost control.

"We're still checking, maybe there was contact between the vehicles, maybe there wasn't, we're still looking at all of those parts of the collision scene to work out precisely what's happened," Inspector Rankin said.

Police have urged the person driving the silver sedan to come forward.

Meanwhile, a backseat passenger was killed when the car she was travelling in crashed into trees at Buangor in Victoria's west on Tuesday night.

Police said the driver lost control after hitting a fox on the Western Highway at 10:30pm near Ararat.

The woman died at the scene and three others were injured in the crash.