“In motoring cases, it is often suggested on behalf of the claimant that drivers should never drive faster than the speed which enables them to stop within the limits of their visibility. Mr Parry pointed out, this would mean, if followed to its logical conclusion, that a typical vehicle should never travel at more than 27 mph with dipped headlights on an unlit road in order to be able to stop within the 40 m or so of visibility likely to be available”