Heart-stopping footage has emerged of the moment an overhead road sign crushed a car on a Melbourne freeway.

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.

                                                                                      “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 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.”Read more at 9news