On Tue, 10 Jul 2007 02:19:04 -0700, MIG wrote:

The trouble is that the preambles aren't true. If you hear "Your
attention please; here is a special announcement ..." you can
guarantee that there will be absolutely nothing special about the
announcement.

On the Jubilee Line, every so often the in-car displays flash
"IMPORTANT" several times (or a similar word, I forget exactly). This
is followed by a message telling you to take your bags with you when
you get off the train.

Of course, the only thing this achieves is to train the passengers to
ignore the flashing notice, so that in the event of actually having to
transmit an important message, this avenue is no longer available.