SSP019 Iss 1

The purpose of approach locking is to prevent the change of route ahead if a signal once the driver has seen a proceed aspect at the signal or has seen an aspect at a previous signal that would indicate to him that the former signal is displaying a proceed aspect. Provision must, however, be made for such locking to be released provided a reasonable assurance can be given that any movement, the driver of which has sighted a proceed aspect, will in the event of the signal being replaced to danger, either have come to a stand at the signal or will have run past the signal onto the track circuits which lock the points or level crossing.