Forget what you learnt at primary school. Four does go into three – or at least it does when it comes to certain motorways.

Transport Secretary Ruth Kelly did her sums last month and announced that the Highways Agency is to add a new hard shoulder "running lane" to two short sections of the M6. This move gives rush-hour motorists four lanes instead of three, and hopefully reduces congestion by 25 per cent at a stroke. The plan follows an 18-month trial on the M42 and a stretch of the busy M25.

At first glance, this is a nice idea. It seems to buy off the environmentalists who object to further road-building on the basis that we're "tarmac addicts" – the more roads you give us pesky car-drivers, the more we seem to want. At the same time, it will play well with rush-hour motorists in the Midlands, who get a widened motorway at exactly the time they need it. Using the hard shoulder will even appeal to the Treasury: new roads cost a fortune, and even widening is expensive.

And could the NHS benefit? Wider roads and free-flowing traffic are surely going to ease the nation's collective blood pressure.

Well, perhaps that's pushing it a bit. But, to be honest, we at the IAM were less than impressed when the pilot schemes were first announced. We just saw potential road safety problems. Hard shoulders are there for the nasty things, like emergencies, breakdowns and clearing the carnage after crashes. Nobody wants them, but hey, these things happen.

We had reservations about impeding the emergency services. How would a fire and rescue service truck get to the scene if it's stuck at the back of a four-lane queue?

We needn't have worried. The trial period showed that overhead gantries are effective in directing traffic away from the hard shoulder if there is a problem, and 95 per cent of the (closely monitored) motorists complied. In fact, there has been a fall in the number of crashes and casualties on the M42. So the hard shoulder is actually needed less often for its original purpose.

Not only are drivers prepared to adhere to a lower speed limit; they have also demonstrated a rather grown-up approach to braking. The M25 drivers on the hard shoulder were less likely to brake sharply, which creates those "phantom" tailbacks that seem to have no cause whatsoever.

And the cherry on the top was another plus for the environmentalists: by getting people to focus on the way they drive, a regular flow of traffic follows rather than stop-start – and smoother progress means lower emissions.

As long as rush-hour speed limits are 50mph or 60mph, there is no problem. Even petrolheads now concede that sometimes going slower can mean that we are all likely to get there sooner.

IAM colleagues point out that being an advanced driver makes a real contribution to reducing our carbon footprints – because crashes are often avoided. Crashes don't just ruin lives, they have a huge environmental impact. Long queues of idling engines stuck behind a careless driver; that's a lot of fuel going nowhere, and zero miles per hour is not good for anybody. And if the M6 hard-shoulder running continues to show that crashes are reduced, then that is indeed a valuable by-product of the plan.

Perhaps the Highways Agency could go further and review through fresh eyes this empty "network" of lanes lying idle most of the time. With active traffic management, wouldn't it be great if those lanes were available more widely? Four lanes mean that motorists are probably less likely to change lanes, which is the single most dangerous manoeuvre we make on motorways.

Until that happens, I am unlikely to find another way to make my journey to work 25 per cent more efficient at no extra cost.

Vince Yearley is head of media at the Institute of Advanced Motorists

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