The Melbourne driver who cheated death when an overhead road sign fell and crushed her car says she cannot believe such an accident could happen in Australia.

Key points: A second sign on the Tullamarine Freeway has been taken down as a precautionary measure

A second sign on the Tullamarine Freeway has been taken down as a precautionary measure An inspection of similar-sized sign and gantries is underway

An inspection of similar-sized sign and gantries is underway VicRoads says an independent investigator has been brought in to determine what happened

Extraordinary dashcam footage shows the moment the five-by-four metre sign fell in front of, and then on top of, Nella Lettieri's car as she was travelling on Melbourne's Tullamarine Freeway earlier this week.

While the 53-year-old was not seriously injured, she is bruised and battered — and wondering how she is still alive.

"It felt like a roller door had slammed shut in front of me," Ms Lettieri said.

"I've gone to swerve, but as I swerved, it just felt like the sign was actually falling on the car.

"And it just kept bouncing, and I felt like it was pushing me to the right, and I'm thinking, 'OK, is it going to stop?'"

Nella Lettieri is surprised — and relieved — to still be alive. ( ABC News )

She thought the metal object may have been from a plane landing or taking off from the nearby Essendon Airport, or from a truck on the freeway.

But she was shocked to discover it was actually an overhead sign, meant to be directing drivers to their destination.

"Yeah, I didn't know what it was at first till I was actually pulled out of there and it was the sign above.

"And I was thinking, 'how did that fall? how did that fall?'"

VicRoads quiet on possible causes

That is a question many are asking, including VicRoads, which has brought in an independent reviewer to get to the bottom of the incident.

VicRoads deputy chief executive, Robyn Seymour, batted away repeated questioning on the topic at a press conference on Thursday.

"It's too early to talk about the detailed findings of that investigation, there's a lot of work that needs to be done," Ms Seymour said when asked about the seeming appearance of rust at the bottom of the sign.

"There's a lot of work that needs to be done with metal specialists and other specialist engineers to really look at the sign, what's happened, look at the footage, really understand the root cause."

On Wednesday one of the directors of Major Road Projects Victoria (MRPV) said the "extremely rare event" of a sign falling on a car seemed to be caused by the sign's connection with the gantry.

But Ms Seymour would not speculate on the matter.

"We really don't want to get into the details of that," she said.

What caused the sign to fall is yet to be determined. ( ABC News )

Inspections of others signs underway

Ms Seymour said similar-sized signs and gantries around the state were being audited, and no issues had been uncovered in the first six audits carried out so far.

"If we were at all concerned about community safety, we would close the road," she said.

"We have deemed there is no risk to community safety at this point."

However the second sign on the gantry at the centre of the incident has been taken down for an inspection as a precautionary measure.

"They have taken down one other sign on that cantilever in order just to satisfy themselves that there's nothing that would have contributed on the cantilever itself," Victoria's Acting Premier, Tim Pallas, said.

Mr Pallas called the dashcam footage a "traumatic and dramatic piece of vision".

"I hope that the person involved is well," he said.

"Certainly the damage that's been caused to the car is quite obvious and the state will pick up the appropriate damages as a consequence through our TAC [Transport Accident Commission] regime."

He assured concerned road users they should feel safe.

"Because people, with a fair degree of reason, would be concerned about signage on that section of freeway, the major road projects authority has overnight inspected all the signs and have satisfied themselves that those signs are in good order," he said.

Sorry, this video has expired Sign falls and crushes car on Tullamarine Freeway

'What are the chances the others will fall?'

But it may be some time before Ms Lettieri drives that stretch of road again.

"I don't think I'm in a hurry to get behind the wheel just yet," she said.

"The scary part is that the whole strip of the freeway, it's all new signs that have gone up, so if that one has just fallen what are the chances the others will?"

She said a "deep investigation" was needed.

"You wouldn't think something like that would happen here in Australia, because we follow specs and procedures.

"It wasn't windy. So something is definitely not right there."

And her advice to other drivers?

"Look up, as well as look ahead."