Motorists are being warned to brace for a horror evening commute after a semi-trailer loaded with boxes of soap rolled on the Western Ring Road.

Two trucks and a car were involved in a massive collision on the Western Ring Road at Glenroy about 10.25am on Tuesday, leaving the semi-trailer loaded with soap lying on its side in the middle of the road.

It brought traffic to a standstill for seven kilometres.

Four Greensborough-bound lanes of the Ring Road were initially closed.

One left lane has since been reopened, but three right hand lanes remained closed after the Pascoe Vale Road exit ramp as at 2.15pm.

The truck will need to be unloaded by hand, due to the weight of the cargo, a VicRoads spokesman said.

"The truck is too heavy to move while loaded," the spokesman said.

Police estimate the road may be closed for up to eight hours, which will wreak further havoc during Tuesday afternoon's traffic peak.

"VicRoads advises motorists to plan their journey home tonight as though the Western Ring Road will remain closed Greensborough-bound, into the evening peak," the spokesman said.

"If you're heading to the airport we recommend exiting the Western Ring Road at Airport Drive, as there are delays approaching the Tullamarine Freeway."

VicRoads said it did not know when the road would reopen, saying the scene was under the control of police.

The crash has created a giant bottleneck in the city's northwest, with delays spilling out onto the Tullamarine and Calder freeways, Melrose Drive and Sharps Road.

Traffic to Melbourne Airport is also heavy and at least one bus service, the route 902 between Chelsea and Airport West, is running 30 minutes behind schedule.

One frustrated road user described the Ring Road as "a mess" on Twitter.

It is unclear what caused the smash and police are asking for witnesses to come forward.

Motorists are being told to exit the Western Ring Road as early as possible and detour via Camp Road before re-entering from Sydney Road.

"VicRoads is working closely with emergency services to get the lanes open as quickly as possible," a statement said.

"We are monitoring traffic on the detour routes and adjusting traffic lights to assist with congestion, where possible."

While Greensborough-bound traffic is having a horror run – it's 50 minutes from the Calder Freeway to Pascoe Vale Road – Altona-bound traffic is running smoothly.

The truck driver was injured in the crash and has been taken to Royal Melbourne Hospital in a stable condition.