Acting Victorian Premier Tim Pallas says the state, through the Transport Accident Commission, will pick up the bill for damages after a horrific incident saw a woman lucky to escape.

Heart-stopping footage emerged yesterday of the moment an overhead road sign came loose and crushed a car on Melbourne's Tullamarine freeway.

All other signs on the freeway have since been inspected with VicRoads removing one as a precaution.

Driver Nella Lettieri (far left) avoided serious injury in the incident. (Supplied)

Mr Pallas says the state's investigation is still underway into what caused the sign to fall.

The video, obtained exclusively by 9News, shows just how lucky the 53-year-old woman behind the wheel was to escape the incident with her life.

Nella Lettieri was driving along Tullamarine Freeway yesterday at around 5.15pm when the five-by-four-metre sign came loose.

The footage was caught on the dash cam of the car travelling behind, driven by Ali Murad.

The woman had no time to swerve to avoid the large sign. (9NEWS)

The moment the overhead sign comes crashing down. (9NEWS)

“She was driving a big SUV, I was driving a smaller car… it would’ve crushed my car,” Mr Murad said.

The aftermath of the incident shows woman’s car decimated, its roof caved in, and glass and debris strewn across the freeway off-ramp.

Emergency services were called to the freeway near Essendon Airport and Ms Lettieri was take the Royal Melbourne Hospital with minor neck injuries.

She remains in a stable condition.

The sign was installed at the end 2017 by CPB contractors under the direction of Transurban and the Major Road Projects Authority. The sign is now controlled by VicRoads, which conducted two maintenance checks yesterday.

“There was no visible sign this was going to occur,” Robyn Seymour from VicRoads said.

A map showing the location of the fallen sign. (9News)

Investigations into the freak accident are ongoing. (9NEWS)

A Transurban spokesperson told 9News.com.au they "undertook initial investigations overnight to check other similar signs installed as part of the CityLink Tulla Widening project.

“Safety is of paramount importance to us and we will continue to work in collaboration with VicRoads, Major Road Projects Victoria and CPB Contactors as part of the investigation.”

Major Roads Project Victoria and Victoria Police are investigating, with engineers to inspect the condition of all similar signs in the next few days.

Major Road Projects Authority delivery director Graeme Chambers earlier told Melbourne radio station 3AW he believed there to be no further risk to drivers.

"Equipment was used to test surrounding areas ... and the experts say there are no concerns that it will happen again," he said.

"We don't know at this moment why this occurred, but we take this very seriously," Mr Chambers said.