Police said the concrete block appeared to be part of a bollard
A driver has told of his narrow escape after a large concrete block was dropped through the sunroof of his 4x4 as he drove along the A12 in Essex.
David Ackland, 48, from Romford, said: "Had I passed a second later it would have hit me and probably killed me."
Essex Police have increased patrols after about a dozen incidents in which concrete was dropped from bridges over the A12 on to passing cars.
Three boys from Chelmsford have been arrested and bailed over the incidents.
A police spokesman said several vehicles had been damaged and two motorists injured.
Essex Police released images of the damage caused to Mr Ackland's Land Rover Discovery as he drove under the Fryerning Lane bridge on 12 November.
Mr Ackland said: "I had just passed under the bridge when a lump of concrete crashed through the roof."
He added: "Thankfully I was on my own at the time, but quite often I have my granddaughter in the back seat and she would have been sitting exactly where the concrete landed.
"I can't bring myself to think about the consequences of that.
"I was on cruise control at 60mph so if the concrete had struck me and rendered me unconscious it could have been potentially devastating for other motorists too."
The block caused £5,000 of damage to Mr Ackland's Land Rover
Det Insp Dean Chapple said: "This particular incident illustrates the severity of the criminal acts we are dealing with.
"Even a stone thrown from a bridge can be fatal, but when you look at the size of this concrete, it is amazing that the consequences were not far, far worse."
The police spokesman said officers were urging motorists and residents to be on the look-out for anyone behaving suspiciously.
They were also keen to receive information about the removal of hardcore or building rubble.
Three boys, two aged 16 and one aged 17, have been released on bail pending further inquiries after they were arrested in connection with the incidents.
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Police say it could only be a matter of time before someone is killed