Colin Rosenstiel wrote to uk.transport.london on Wed, 17 Nov 2004:

I'm not so sure about that. Speaking from over 25 years working for the
company that made the radios for Tramlink and other PT, distinguishing the
message type is pretty trivial technology probably already incorporated.
I'd be more worried about the voice quality being good enough for PA over
the radio, though.

A lot of modern buses do seem to have PA incorporated, so that the
driver can make announcements - notably, of course, the one who was
featured on the news last Tube strike telling her passengers that she
would open the door when it was safe to do so! I imagine it wouldn't be
beyond the realms of possibility for PA to be standard on all buses, so
that drivers could relay information given over the radio.
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 6 November 2004 with new photos