"Brimstone" wrote in message
...
Paul Terry wrote:
In message
,
MIG writes

[ blue lights on street lamps]

A wild guess: maybe it's to do with distinguishing between power off
and the lamp failing?

That's possible, but I posted here a long time ago about new street
lamps on the South Circular that have a very distinct (and not at all
feint) blue lens pointing upwards.

This is very obvious from above, but can only be seen from ground
level if standing a very long way from the lamp post concerned.

If it is some sort of pilot lamp, it seems very badly positioned.

I didn't receive any plausible replies when I posted about this, but I
have seen it suggested that modern "cobra" street lamps are so well
designed to prevent night-sky pollution, that they need an upward
aspect in blue in order to delineate the location of major roads for
aircraft that, due to failure of automatic systems, are having to
rely on visual navigation at night.

Lit roads are perfectly visible even if you can't see individual lamps.
Besides, there are usually much better markers than roads - sports stadia
are very distinctive, for example.

....
Possibly they're markers for police and other emergency helicopters?

That sounds the most likely. They would want to know where the posts are
when trying to land on a road.

Colin Bignell