A missing steel plate used to stiffen gantries may be the reason a massive sign crashed down on a woman’s car in Melbourne.

Preliminary findings shared on Sunday reveal a stiffener plate had not been installed in the gantry, leading to the sign falling on to Nella Lettieri’s car on the Tullamarine Freeway on January 8.

“The preliminary findings have determined that the failure has occurred at the connection of the vertical structural sign supports to the horizontal gantry beam,” a VicRoads statement reads.

“The investigation to date has identified that a ‘stiffener plate’ had not been installed as per the approved design.

“It is important to assess all potential contributing factors and consider the full findings of the investigation before making a definitive statement on the cause.”

EXCLUSIVE: This incredible dash cam vision just in to the @9NewsMelb newsroom shows the moment a giant road sign plummets from a gantry crushing the 4WD below. Somehow the 53 YO woman escaped serious injury, expected to be discharged from hospital today. pic.twitter.com/3PwxpQeovV

— Lana Murphy (@LanaMurphy) January 9, 2019

The steel plate is inside a gantry to strengthen the connection of the sign structure to the overhead framework.

An audit of 17 gantries with similar sign support, installed by the same contractor as part of the CityLink Tullamarine Widening Project, found plates missing in two other gantries.

While there were no signs of stress or deterioration, they were removed last weekend.

Stiffener plates were not required to support any of the other similar signs installed in the same project, VicRoads said.

The investigation still under way is also considering “engineering and design calculations and materials testing” as to determine if there any other potential contributing factors.

VicRoads is undertaking a precautionary audit of all top-mounted static signs on similar gantries across Melbourne’s freeway network with none showing stress or deterioration.

Ms Lettieri escaped serious injury but the weight of the sign crushed her car, shattered the windscreen and scattered glass across the highway.

A huge overhead traffic sign has fallen from its frame, crushing a car on Melbourne's Tullamarine Freeway. @EstelleGriepink #TheLatest #7News pic.twitter.com/Z5MXsw3AOx

— 7NEWS Melbourne (@7NewsMelbourne) January 8, 2019

-AAP