Madness as fog thickens on the M4
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A TRANSPORT and Road Research Laboratory study of driver behaviour in fog has shown that when visability was more than 100 yards most drivers kept intervals which would have enabled them to stop safely in dry conditions.
But, when visibility dropped below 50 yards, the distance between vehicles in many instances dropped below the safety margin.
The results, from a study on the M4 at Theale, Berks, have also shown that many drivers were exceeding the legal speed limit.
The study was carried out with the help of ORBIS III equipment which included a camera capable of photographing the rear ends of vehicles in fog.
To date, only the second and third lanes of the motorway have been monitored in this pilot experiment_ The equipment could only cover one lane at a time and there were insufficient periods of fog to investigate the nearside line.
Pairs of axle sensors buried in the road surface were activated by a passing vehicle.
These sensors work on the first two axles only with a subsequent delay in the timing to ensure that any more axles (for example a trailer bogie) do not confuse the issue.
In fact, because the nearside lane has not yet been observed, very few heavy commercial vehicles have been picked up.
When the visibility was more than 100 yards, it was found that the overall speeds were high — 101 and 116km/h (63 and 72mph) in the second and third lanes respectively — but the equivalent Highway Code stopping distances were within the prevailing visibility.
However, when visibility was 50 yards or less, the mean speed of the traffic was too high to enable the drivers to stop within the distance visible ahead.