Rajesh Kakad:
So we should be able to turn LEFT at a RED light.

Richard Willis:
In general, I agree, but we do have a sort-of system where this is
allowed: green filter arrows.

... Do the USAns have filter arrows, or is it a Europe-specific thing ?

In North America, arrows are mostly used for protected left [= UK right
:-)] turns -- that is, the left-turning traffic has the right of way,
and all conflicting traffic has a red light. Perhaps the most common
way this is used is for straight-ahead traffic to have a red light in
all directions, while left turns in both directions from one of the two
streets have a green left arrow; in some jurisdictions a yellow arrow is
used to warn of the end of this phase; it is typically, but not always,
followed either by the regular green or by a green light that does not
apply to left-turning traffic. It works best when there is room for a
a separate lane to be designated for left-turning traffic approaching
the intersection.

I haven't driven enough in British cities to know whether the mirror-
image of this is a common pattern there.

Aside: does anyone know why some traffic signals here show a
green filter arrow *AND* a solid green light simultaneoulsy,
given that the latter allows a superset of the filtered traffic
to "go". ?

In North America, the combination of a green left arrow and an ordinary
green means that all moves are permitted, but the left turn is protected.
In Ontario and several other Canadian provinces, a flashing green is used
instead of this combination, with effectively the same meaning; but this
aspect is now being phased out, at least in Ontario. (Other meanings of
flashing green exist in other places, notably the province of British
Columbia.)

This response may, of course, be completely irrelevant to Richard's
question. If the meaning of the signal combination really is exactly
the same as the regular signal, maybe it is used just in case there are
people who think it might not be, and would not turn when the arrow was
dark.
--
Mark Brader, Toronto "C and C++ are two different languages.
That's UK policy..." -- Clive Feather

My text in this article is in the public domain.