For a European, North American colour light signals seem to be three lights that can display any possible combination of colours and flashing lights - without making too much sense.
Well, if you look at the fact that almost every railway company has at least one signal system, or more than one if it is a merger, and that there are things that they call position light signals, which can display colourful lights (while the colour has no meaning, just the arrangement of lights), you start wondering.
But then you learn that a basic rule is "if it's not all red,
it's not red at all1",
i.e.other than the European convention that a red is stop and never
anything else, in
North America a red is just the colour used if a light has nothing
useful to display.
In other words, red is the colour that a lantern shows to display "disregard me - I'm not really here", you stop wondering and conclude that their signals don't make sense at all (except keeping anyone below Einstein's IQ from passing the drivers test).
Now, if you take a deeper look, some things in fact make
(some) sense, and here I will
explain what I think is the secret.
As examples I use the aspects and rules as used by the Canadian railways (CROR-Canadian Railway Operating Rules). Most signals display the speed to be used after this signal and indicate the aspect which the next signal will show, similar to German combination (Ks) signals, Deutsche Reichsbahn's Hl signals or a combination of Hp home and distant signals.
In the table below, the lanterns that are displayed in dimmed colours may be absent. If present, they display a red light (as I mentioned above: if it's not all red, a red just says "ignore me"). Usually you can ignore any red from the bottom up until you meet the first non-red. Since that would render the system uselessly simple, they have exceptions: see below.
Four aspects immediately seem to be logical: A green for clear, an amber for caution-expect stop, and a red for stop; as well as a flashing amber for advance warning.
For the aspects that have a red on top, remember that before points, the British used to have semaphores with different heights, the higher one for the straight track and lower ones for diverging tracks. So if a diversion was to be announced, the driver would see a red on top, and to the lower side a green. This system is similar, although the British signals carry no speed information but are path signals only.
An amber on top (or green over red, not shown in the table) tells you that the current speed is clear, and the next signal will show clear with a speed limit (but neither clear nor stop).
A red on top tells you that the current speed is not maximum. The speed after this signal is given by the second lantern from top, and the next signal's indication is shown by the third lamp, unless the next signal show clear or stop.
In the former cases, a green indicates medium speed, a flashing green indicates limited speed, and a flashing amber indicates slow speed. Examples: red-flashing green-green means "limited to medium".
Exceptions are the indications "something to clear" and
When the next signal shows clear, the third lamp is red (:-0) (so here red means "expect clear", and this page's caption seems to become true...), or in the case of "slow to clear" the aspect is shown using the second and third lamp, if a top lamp is present displaying the "filler red", the aspect becomes red-red-green.
When the next signal is at stop, the same aspect as for expect clear is used, just using an amber instead of the green, which makes it incompatible with the other aspects. You got that? (It took me a number of sheets of paper and a bunch of brain cells)
So most of the aspects seem to make sense, although the use of a red lantern for either showing "I'm not here" or "expect clear" is something that would take some time to get used to...
Signals can have one or multiple heads, and the rule is that no head must ever be completely dark. As example, a signal with three lamps may have one or three heads. So the aspect "clear, expect stop" is shown by one amber light if the signal has one head, if it has more than one, e.g. two or three, the bottom head(s) display red:
|all these say "clear, expect stop"|
Ah yes, since it would be too easy just to tell the speed by
numbers, they call the
speeds names: Instead of just saying 24 km/h,
48 km/h, or 72 km/h they say
things like "slow", "medium", or "limited speed".
By the way, "restricted speed" is about the same speed as "slow", just be on the watch and prepared to brake short of other vehicles in your track, so according to European usage a "restricted speed" is not a speed signal but some sort of caution signal.
A good site about different signalling systems, including US, is that by Mark Vogel
1) "If it's not all red, it's not red at all", see on Mark Bej's site on learning NORAC aspects.
|U-Bahn Berlin||OSShD signals|