A million-dollar high-technology warning system will be installed at a notorious bridge in Melbourne's west, to stop trucks and large vehicles from crashing into it.

The Napier Street bridge in Footscray has been struck more than 70 times in the past 12 years.

Every time the bridge is struck the trains on the Werribee line are stopped and the structural integrity of the bridge is checked.

A new $1.2 million detection system will be installed to alert drivers if their vehicles are too tall and direct them to another route.

Laser beams will trigger flashing warnings and the stop lights will turn red to prevent the truck from proceeding.

Roads Minister Luke Donnellan said the upgrade was sorely needed because despite 28 height warning signs on the approach to the bridge, it was still being hit.

"You've really got to be a dickhead in ignoring every sign to hit a bridge," Mr Donnellan said.

"This is a more activist intervention to literally say 'well if you are going to hit the bridge you will have had to ignore traffic signals'.

"You would have had to have gone through red lights and you would have to deliberately defy everything along the way to actually hit the bridge."

Long flaps hang from the gantry to warn trucks before driving under the Montague Street bridge. ( ABC News: Justine Longmore )

'Just plain defiance'

It is also a problem at Montague St, South Melbourne, where gantries with hanging flaps were installed to stop tall trucks from getting stuck.

"Even the Montague St bridge, [where] we've got creepy crawly spiders coming all over the truck before they hit the bridge and people still ignore that," Mr Donnellan said.

"I mean I don't know what you do sometimes. It's just plain defiance."

In February 2016, a driver and 11 passengers were taken to hospital when a charter bus, which was 3.8 metres high, slammed into the three-metre high bridge.

Mr Donnellan said laser technology was now cheaper in price and that had allowed government to trial it at this intersection.

He said the new system will be as strong as it can be apart from stationing a police officer at the intersection.

"You've really got to be a driver who's just not paying attention and obviously has no interest in safety," Mr Donnellan said.

"It's not good enough that people continue driving trucks and ignore the messaging."