I suspect I'm going to get in trouble for incorrect usage of the word
'differential', but nevermind...

I'm hoping that someone in the know (probably Paul C, given that he
was involved in the development of Oyster) might be able to provide
some background to this query.

As we all know bus fares in London are flat rate - Oyster PAYG users
pay 90p, cash fares are £2 - and the flat fare for London buses has
been around since 2004 (I think). There were peak (pre-0930) and off-
peak Oyster PAYG fares on buses for a period - but there have never
been different fares according to distance travelled.

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.

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
has to enter in into their ticket machine before the passenger then
touches-in their Oyster on the ticket machine's Oyster pad - and are
possibly also issued with a paper ticket.

Please note that I'm not advocating anything like this whatsoever - I
think it would be a total and unmitigated disaster as it would
completely negate the advantage that passengers could board the bus
quickly. However I'm interested to know whether the Oyster system is
at all capable of such a thing, or whether the presumption that bus
fares would go flat rate was built into Oyster from the start.

I ask prompted by thoughts of how, if and when stored value ITSO
smartcard ticketing finally arrives on buses elsewhere in the UK, how
the situation with regards to fares