"Peter Masson" wrote in message
...

Presumably the signal itself uses an LED display, so that the upper head
can
display green or yellow, and the lower head yellow or red, and the signal
as
a whole can display all four aspects R, Y, YY, and G.

I think I read somewhere that both heads can display red, but that normally
the lower one is used for red. The upper head can be used in emergency to
display red if the lower one has failed for any reason. BICBW.

Regards

Jonathan