In message , Colin
writes
The Thames bridges also have "traffic lights" for vessels, so this is
actually a SPAD as well!

Sort of - see the 'Tower Bridge Signals' PLA diagrams showing Bridge Lights
lower down the page he

http://www.portoflondon.co.uk/displa...&site=maritime

"Isophase lights are exhibited on certain arches of the bridges from
Tower Bridge to Putney Bridge to warn navigators of the imminent passage
of large vessels."

[These seem to be electronically activated by the larger boats carrying
what amounts to a transponder. It's many years since I've driven down
the river by boat, but I think there used to be things much more like
conventional "traffic lights" performing the same function.]

And then there's the lights you mention that change from Red to Green
when tower Bridge is open.

ps The Interactive Leisure Map on the Port of London site is very
interesting, as it shows railways (and roads) as well as the river.
Including the new Channel Tunnel link.
--
Roland Perry