The collapse of an overhead Melbourne freeway sign onto a passing car, with the driver lucky to escape serious injury, has prompted an inquiry.
The state government and roads authority on Wednesday said the matter would be thoroughly investigated but said such occurrences were rare.
A woman was taken to Royal Melbourne Hospital with an injured arm after the four-metre by five-metre metal sign toppled onto her vehicle on the Tullamarine freeway, near Essendon Airport, about 5.30pm on Tuesday.
Footage from the scene shows a blue car crushed by the sign, with its windscreen shattered and glass strewn across the road. No other cars were hit.
Graeme Chambers, delivery director at the Major Roads Project Authority, said a detailed investigation is under way.
"This is an extremely rare situation," he told 3AW on Wednesday.
"It's somehow failed where the connection is to the overhead gantry.
"We last night mobilised our engineers as soon as we learnt of the incident.
"Obviously our thoughts are with the woman as she recovers."
Mr Chambers said other signs installed about a year ago as part of the same Transurban freeway widening project would be inspected to ensure they were safe.
A neighbouring sign on the same gantry was inspected on Tuesday night, and experts found no problems, he said.
"I'll be getting further updates today to understand why this has happened. We obviously take this very seriously," he said.
Mr Chambers said the authority was working with Transurban, their contractor and VicRoads as part of the probe.
Government minister Ben Carroll said it was concerning the sign fell.
"I cannot underscore how rare that occurrence is," he told reporters, noting he uses the road almost daily.
"This sign was put up as part of the CityLink Tulla Authority work that went out there so they have given us confidence that the additional signs put on that gantry are safe and secure."
Additional gantry fixtures would be investigated, he said.