On Thu, 6 Dec 2007 00:14:41 -0800 (PST), PhilD
wrote:

On Dec 5, 11:13 pm, Barry Salter wrote:
Can anyone tell me if this is in use? How does it work? Are there
others? Etc..

The short answer is that it's a SPAD (Signal Passed At Danger) Indicator.

snip Barry's explanation

There are others dotted around the country. I know of some at
Reading. They tend to appear in busy/cluttered areas where risk of
over run is higher (or at least, the consequences could be worse).

PhilD

Several around the west end of Leeds as well.

Thanks for the explanation - it's a question I've been meaning to ask
for a while myself!