Several compulsory safety checks were not carried out on a road sign before it fell and crushed a car on a Melbourne freeway earlier this year, an investigation has found.

The final report on what caused the Tullamarine Freeway sign to fall uncovered "weaknesses in quality control" in the design and construction process.

The five-by-four-metre sign landed on the road and fell back onto Nella Lettieri's car, crushing the front of it, as she was driving along the freeway in January.

The final report has not been publicly released in full, but VicRoads said it confirmed the sign's fall was caused by a "progressive fatigue crack", which was the result of a vital part not being installed with the sign.

It was missing a steel stiffener plate, which should have been inside the gantry and is designed to strengthen the sign's attachment to the gantry.

"The investigation considered the full design and construct process to understand how the stiffener plate omission occurred," VicRoads' statement said.

"It found weaknesses in quality control and that several required inspections were not carried out."

Ms Lettieri suffered neck and hand injuries in the incident, which she described as "like a roller door had slammed shut in front of me".

It prompted a forensic examination of freeway signs across Melbourne.

CPB Contractors carried out the investigation into its own project, but it was independently verified, VicRoads said.

The driver was lucky to escape the crash without serious injury. ( ABC News )

Freeway signs to be strengthened

Earlier investigations found two other signs were also missing the plates, and they were removed.

CPB Contractors — part of the multinational CIMIC Group, formerly known as Leighton Holdings — had "already taken steps to ensure it cannot happen again", VicRoads said.

They include:

Increased surveillance of offsite subcontractors and suppliers

Increased surveillance of offsite subcontractors and suppliers "Improved processes" to ensure subcontractors show that fabrication, construction and installation follow approved designs

"Improved processes" to ensure subcontractors show that fabrication, construction and installation follow approved designs "Hold points" in the design and construction processes to ensure vital quality and safety elements are verified before works continue.

Private toll road operator Transurban would also strengthen top-mounted signs installed as part of the CityLink Tulla Widening project, between the Bolte Bridge and Tullamarine Airport, so they would last at least 100 years, VicRoads said.

"VicRoads, Major Road Projects Victoria and Transurban have audited similar signs on the Managed Motorway network and are confident this is an isolated incident," VicRoads said.

"There was no evidence of stress or deterioration on any signs audited."

CPB Contractors is working on Melbourne's multi-billion-dollar West Gate Tunnel project, Sydney's WestConnex projects and major roads in Queensland and Western Australia.