Commuters faced heavy delays after two separate car crashes forced lane closures in both directions on the West Gate Freeway on Tuesday.

A motorcyclist and truck collided outbound about 12.45pm on Tuesday, and the rider was taken to hospital in a critical condition.

Police said they were trying to locate the truck driver and the exact cause of the collision is yet to be determined.

Four outbound lanes were closed for hours and reopened around 4.30pm.