by Chris Kelly BBC News, Bristol

But from Wednesday, Wiltshire will see mobile cameras operating from fixed, permanent sites along the road.

Speed cameras will be used to enforce the speed limit on the M4

From now on, if you drive over the speed limit between J18 at Bath and J14 at Hungerford, you face a �60 fine and three penalty points if caught.

Those fines end up with the Treasury and the Department for Transport, which gives permission for cameras at accident blackspots.

The Wiltshire move is already proving controversial.

At camera sites in roadworks along the M4, an early problem became apparent.

Panic braking

Day after day, drivers have been sailing up to the cameras and slamming on the brakes as soon as they saw the distinctive white lines painted on the road.

Paul Smith, of road safety campaign group Safe Speed, warned that panic braking could be a problem.

"They absolutely will not work to improve road safety," he said.

"Apart from the distraction and panic braking effects we also have to worry about moving drivers outside their 'zone of optimal performance'.

People panic braking just shows they are bad drivers Saira Khan, Wiltshire Safety Camera Partnership

"After �700m of speed camera fines we've seen no reduction in general vehicle speeds and no improvement in roads fatalities."

But the blunt message from the Swindon & Wiltshire Speed Camera Partnership - which is running the scheme - is that 240 people died or were seriously hurt on the M4 in Wiltshire last year - and it wants to reduce that toll.

Spokeswoman Saira Khan told BBC News: "If those 240 people had been involved in in one incident in one day there would have been an outcry."

HAVE YOUR SAY It is only changing drivers' attitudes that will make a difference Anon, Swindon, Wiltshire Send us your comments

The rate of deaths has jumped dramatically in the past five years.

In 1999, 193 people lost their lives or were seriously injured - today that figure has shot up almost 25%.

'Bad drivers'

"At all our camera sites in Wiltshire we've seen a 64% drop in personal injury collisions. If we can achieve this on the M4 it would be great," Ms Khan added.

"People panic braking just shows they are bad drivers - everyone knows what the speed limit is. What does it tell you? They should have slowed down, they are not paying attention.

"If something goes wrong at higher speeds it often means the accident can be fatal. Newer cars aren't good enough to stop you from being crushed."

Crashes on the M4 in Wiltshire happen almost every day

Paul Pinkington, a public health lecturer at the University of the West of England, recently carried out a study into the use of cameras.

His worked showed that they can cut casualty rates by up to 70%, but says they might work differently on motorways.

"The important thing is to make sure they are monitored," he said. "They need to make sure they are being used in the best way, like at accident blackspots."

'Money-making'

Motorists at Leigh Delamare services, on the M4 near Chippenham, seemed unimpressed at the plans, with one claiming they were simply a way of raking in cash.

"It definitely is a money-making business. There's no doubt about it. They are putting them up everywhere just to make money," one said.

"Are the police doing anything these days? No, Not really they're just putting the cameras up to save them more work really."

But another said motorists should stick to the national speed limit.