Speed cameras that track a motorist's average speed along a stretch of road are being introduced on the M6 near Preston.

They will operate between junctions 32 at Broughton and 33 at Galgate for 23 weeks while roadworks are completed.

The Speed Check Services system, known as Specs, sparked controversy when it was introduced in other areas.

They were dubbed "yellow vultures" because of their colour and the way they are perched on poles.

Unlike conventional speed cameras, the £170,000 Specs system works by measuring and recording the speed of vehicles as they pass through the entire length of the roadworks site. If a motorist's overall average speed exceeds the limit, normal penalties will apply.

The M6 camera, the first to be used in Lancashire, will be switched on next Wednesday, and will be used while a footbridge is upgraded to a bridge for vehicles. Lane closures and a 50 mph speed limit will also be in place while a plastic bridge is built on the side of the carriageway.

Bosses at the Lancashire Road Safety Partnership denied the cameras were just a money spinner.

John Davies, project manager, said: "It's purely for the safety of the road workers and also to protect road users. What we are trying to do is slow people down throughout the whole of the road works, as these cameras measure an average speed they are better able to do that."

Latest figures from the Highways Agency show a sharp increase in road worker deaths in the first half of this year - four workers died compared to one in the whole of last year, and two in 2003.

It is also hoped Specs will eliminate so-called camera surfing, preventing incidents of tailback shunt type collisions, and smoother traffic flow.