1. Where barriers are used, they shall when lowered across a private road or path be long enough to extend either—
(a)across the full width of the private road or path, or
(b)from the left-hand side of the private road or path (as seen from a vehicle approaching the crossing) to a point as close as possible to the centre of the private road or path as is consistent with allowing a clear passage at least 3 metres wide between the tip of the barrier and the other side of the private road or path.