M.I.G.:
On the top deck of double deckers there used to only be one bell push,
at the top of the stairs.

Neil Williams:
Some buses used to have a strip on each side reachable from every
seat. In that regard we've gone backwards...

In most North American city buses that I've used, there's been a cord
running the length of the bus on each side, typically through metal
loops at (or below) the upper corners of the main windows. So only two
electrical switches at the front of the bus are required. Depending on
the layout around the back doors, there may be a separate cord behind
the doors on that side, requiring just one more switch.

It seems a very good method to me: cheap, simple, and effective.
The only downside is that passengers seated by the aisle have to lean
across one person to reach the cord. (And if there is a section where
the cord can't be reached easily for some reason, pushbuttons can still
be provided there. Or short sections of vertical cord anchored at the
bottom can be attached to the horizontal cord.) In Toronto these days
the cord is plastic-covered metal for durability; at one time it was an
ordinary cord.

Is this method used in Britain to any extent?

In both of these shots -- one a Toronto bus previously used in Montreal,
the other from Livermore, California -- you can clearly see the cord on
the right-hand side of the image, in front of the bars between the main
and upper windows:

http://transit.toronto.on.ca/images/bus-8502-03.jpg
http://www.lavta.org/about/img/avlpic14.jpg
--
Mark Brader, Toronto | "We don't use clubs; they weren't invented here.
| We use rocks." -- David Keldsen

My text in this article is in the public domain.