Tristán White wrote:
I took this photo:

http://www.flickr.com/photos/25124954@N00/480556440/
#comment72157600218892777

Can you guess...

Before reading the other replies...


(a) where it is

Warren Street, bottom of Northern Line escalators

(b) what it's for?

I've been meaning to post this question for bloody MONTHS and keep
forgetting!

I know the answer to (a), of course, but would love to know the answer to
(b) as it confuses the hell out of me whenever I see it.

Me too.