Just to note that there are other forms of "delayed" aspect changes. One such example, is in a four aspect area where there is insufficient service braking distance between what would normally be a yellow aspect and the next signal that is red. So the signal that would normally show yellow will stay red when routed by the signaller and only clear to a yellow when the interlocking detects a train approaching the signal in rear. Hence the driver of an approaching train will potentially see:
First signal signal showing green.
The next signal showing double yellow.
The next signal showing a single yellow, which steps up to a double yellow as the train approaches it.
The next signal showing single yellow.
The next signal showing red, which may clear to a proceed aspect if and when the signaller has routed it.
There are other examples, such as signals that form double blocking (two consecutive red signals protecting a junction) as well as other variations.
The principle behind the signalling used in this country is that drivers obey the displayed aspect and don't assume or guess what happens next.
Many drivers have been caught out by forming a habit because they do the same move and have the same aspect sequence every day. Until of course there is the one occasion where something different was happening (or not happening as per usual). Then they don't get the aspect sequence they expect but still drive as if they did... Now I don't want to be alarmist, as the vast majority of mistakes don't result in any substantial increase in risk.
This was a such a common problem in the 1970 re-signalling of Bristol Temple Meads that a special monitoring and alarm system was wired in to detect the most common problem (drivers going beyond the special X signals/signs when the gantry signal had not displayed the appropriate aspect for the far end platform and then stopping in the far end {wrong} platform when they were signalled into, and should have stopped in the near end platform).
Modern computer based interlockings and relay based interlockings that have had a monitoring system retrospectively installed can identify many of these driver errors.
I'm not saying this to scare or attack drivers, far from it, 99.99% of drivers are excellent at their job. But am saying this to help understanding of how the signalling system works.
But as the well know expression goes, it's better to be prepared. And that includes knowledge.
I've always been of the opinion that every single driver should have the opportunity to visit a PSB and or a Signalling Centre, or if they mostly drive in a mechanical signalled area, a traditional signal box, so that they can see what signallers do and how the world looks to a signaller. Plus, and this is important, they can see how the signals respond to the signallers controls and, for electrically operated signals, how they respond to the movement of trains.
Ideally, at least some drivers should also be given an opportunity to see a signalling equipment room. To see "the guts" with a signalling technician present, so that they can give an overview (without getting too technical) of what an interlocking actually is.
