Transport experts have called for changes to the layout of CityLink, after a horrific crash in which a truck driver fell nearly 20 metres from an overpass.

Friday's crash between a truck and car on an overpass between Flemington and Footscray roads caused widespread gridlock after emergency services closed the southbound lanes of the freeway for nearly 12 hours. The lanes were reopened in stages in time for the evening peak, but there were still some delays well into the night.

Police said the car was travelling towards the city and attempted to merge in front of the truck about 6.30am. The car made contact with the truck, spun and landed in front of it. The truck then swerved, hit the right barrier, veered across four lanes and landed on the left concrete barrier.

The 39-year-old truck driver, from Roxburgh Park, was thrown from the cabin, plunging to the ground below. He was in hospital in a critical condition with head and pelvic injuries. The driver of the car, a 25-year-old Spotswood man, was in hospital in a stable condition.