A hooligan who cracked the windscreen of a lorry carrying highly explosive gas canisters by hurling a bottle from an overhead bridge has walked free from court.

Alan Johnston was ordered to carry out 100 hours community service after being told the incident could have caused "devastation" on the motorway.

Trucker Kevin Robb described how he was carrying the explosive cargo of 144 gas canisters when a Lucozade bottle was thrown from a flyover into his windscreen.

He said he heard a loud bang as the object smashed straight through the truck's fibre glass sun visor and then cracked the centre of the windscreen.

Mr Robb, 45, told Perth Sheriff Court: "If it had come through the windscreen or come through a split second later there would just have been devastation.

"I saw the two youths walking across the bridge. The one with the striped top went over the railing. I saw him lifting his hand and I was just waiting for the impact. Then I heard a loud bang.

"Something was thrown from the bridge after I went underneath it. The visor in the middle of the windscreen shattered as it went through it and there was a mark down the middle of the windscreen. It was something quite large to have caused that damage. The sun visor had a hole right through the middle of it."

Alan Johnston, 29, of McBain Place, Kinross, and his female friend admitted they had been on the bridge but he claimed he had dropped the bottle by accident.

During a police interview, Johnston said: "I was on the bridge over the motorway earlier. A bottle of juice - a plastic bottle of Lucozade - was knocked off the bridge. It was sitting on the rail. I knocked it over with my arm."

Giving evidence, Johnston said that he had been pressured to admit throwing the bottle in order to get his case processed more quickly. He admitted downing a bottle of vodka during the evening before the incident which happened in the early hours of the morning.

However, Sheriff Michael Fletcher said that he preferred the evidence of the Crown witnesses and found Johnston guilty of the offence.

The court was told by an expert analysis that a plastic bottle of Lucozade with a small amount of liquid left in it would have been heavy enough to smash through the visor and break the windscreen.

Sheriff Fletcher told Johnston that if it had been proved to have been a heavier object he would have been sent to prison instead.