Mizter T wrote

The fact that there is a flat fare makes implementing Oyster PAYG on
buses very easy - everyone who touches-in (i.e. validates their Oyster
by scanning it) pays the same standard fare (though yes, daily price
capping may mean that they don't!).

However I was just wondering if the system had been designed so that
it could be used in a non-flat fare environment - i.e. fares based on
distance or zones. I cannot envisage a scenario where passengers had
to touch-out when they exited the bus as being remotely workable
whatsoever, so that's not really what I'm asking about.

Posters here have in fact noted that many European cities DO require
bus users to touch-out on exit.

Instead the situation I have in mind is one where the passenger has
to
specifically request a destination (or zone) to the driver, who then
[...]

Most cities and PTE areas in the UK are smaller than the GLC so flat
rate in a central zone seems likely. I am sure the NG experts will
comment.

However when the charging scheme also has to work for supertram and
express services (greenline or Oxford tube in London terms) something
more is needed.

If these are limited stop and since the cost of "pay before you board"
machines is apparently tolerable a U'ground type scheme seems possible
with validators at stops.

Assuming standard buses are flat £1 fare, the card would have say £5
deducted when the user boards and touches-in and if the journey
finished at a express stop or tram stop within the city touching-out at
a stop validator would return value so the journey would cost from £1
upwards, perhaps with a minimum fare of £2 for express buses. If the
journey finished outside the city where there were no validators then
the maximum would apply. For added complication a discount for a
subsequent return journey could be allowed, even on the following day.

--
Mike D