When truck driver Graham Kennett allegedly ploughed into Penrose Rd overbridge, the warning system was dismantled and replaced with a "vastly inferior" sign, a court has heard.

And of the 12 other drivers who have clipped the bridge, Kennett is the only one to be prosecuted for dangerous driving, his lawyer said.

Kennett, 65, is on trial for dangerous driving after the crash on State Highway 1 on May 9 last year, which caused hours of gridlock across Auckland.

The Taupo man has accepted his load was oversized but is defending the charge in a judge-alone trial at the Auckland District Court on the grounds the warning system at the time was inadequate.

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Kennett's lawyer, Simon Stokes, said the warning system had been replaced with a "vastly inferior" sign 250m away from the bridge.

And of the 12 other drivers who also clipped the overpass with oversized loads, 10 were only given an infringement notice and two were charged with careless driving.

However, Kennett is the only one who has been prosecuted for dangerous driving, Stokes said.

A man driving southbound in the lane next to Kennett's truck and who narrowly missed being crushed by the falling digger described the moment the crash happened to the court.

Hayden Bishop was in the right-hand lane and remembered seeing a dump-truck with a flatbed carrying a digger in the lane to his left.

"I heard an almighty bang and I had to react. I was thinking in my head straight away, do I stay or do I go. I just accelerated through everything."

Police prosecutor Sergeant Sam McErlean then showed the court a number of videos with different angles of the overpass strike.

The bright yellow digger can be seen slamming into the overpass in a cloud of dust and the machine falling to the road, narrowly missing Bishop's car.

McErlean asked Bishop to describe what he thought after seeing the videos.

"I suppose shock. You're not thinking much because you're like what's just gone on and are sitting there trying to process what's happened."

Under cross-examination by Stokes, Bishop said he remembered seeing a "temporary" sign on the left-hand side of the motorway, which was different to the normal warning signs he was used to.

Auckland Motorways Alliance engineer Zane Davidson told the court an earlier strike to the Penrose overbridge in March damaged an LED display sign.

The "variable message sign" could alert drivers if they had an overheight load but it had been replaced with a temporary sign.

The defence also established that variable message signs on two bridges before Penrose had also been dismantled at the time of the May 9 crash.

However, the Auckland Motorway Alliance wasn't required to have the variable message signs; they were there for bridge protection.

"In my view, they're over and above what is required."

The trial before Judge Grant Powell continues this afternoon and is set down for one day only.