It sounds like science fiction, but a huge magnet is sweeping over motorways in the district.

A specially modified truck has been introduced to clear the hard shoulders of the M606 and M62 of metal debris and nails.

The vehicle, which has a large bar magnet mounted on it, has only been used by the Highways Agency in one other part of the country before, will follow the normal sweeper vehicles on patrol.

West Yorkshire Police, whose patrol cars often suffer punctures when using the hard shoulder, asked for it to be brought to the region in a bid to cut thousands of pounds off their burst tyres bill.

Sergeant Ian Symonds, of the Motorways Unit, said: "The problem was compounded by the fact that because our patrol cars often have to travel at high speeds, we have a policy of replacing, rather than repairing, punctured tyres, at a cost of £15,000 a year.

"We are hoping this magnet will significantly reduce the amount we spend each year on replacing our tyres."

Police have to use the hard shoulder in emergencies to reach accident scenes but often suffer blow-outs caused by sharp pieces of waste left over from previous accidents and breakdowns.

The device, which proved very effective when it was piloted in Hereford, is also being used on the M1 and M18, and the Highways Agency predicts that there will soon be huge mobile magnets roaming up and down motorways all over the UK.

Information officer for the Central Office of Information in Yorkshire and the Humber, Alice Insley, said: "Hopefully it will catch on because this is the sort of problem we get up and down the country.

"West Yorkshire Police approached the Highways Agency about the problem of burst tyres and asked if we could do anything, and this was the unusual solution.