September is the time for mists and fogs. The nights are turning chilly and in calm conditions with hardly a breath of wind, the cold air can turn moisture in the air into banks of mist and fog, sometimes with devastating results. Thick fog 40 years ago led to a catastrophic multiple vehicle crash on the M6 at Thelwall, near Warrington on the Lancashire/Cheshire border.

On 13 September 1971, some 200 cars, trucks and tankers piled up, five vehicles burst into flames, 10 people were killed and 70 injured. It was the worst accident recorded on British roads. One ambulanceman described the scene: "As soon as we managed to free one lot there were cries and shouts from other cars. It was just like a battlefield: wrecked cars and lorries everywhere." A Cheshire police official remarked: "People were just driving too fast for the conditions. This is motorway madness."

Motorway madness seems to be caused when drivers become disorientated and lose their sense of speed and distance in fog. All sorts of measures were tried out to reduce the accidents, with the introduction of the 70mph speed limit on motorways, followed by metal barriers, overhead lighting and hazard warning signs, but with little success.

One remedy was found accidentally, though. The cleaning up of coal smoke pollution to prevent the old pea-souper smogs also led to a sharp reduction in fogs, and fewer pile-ups on the motorways, over the past 50 years.