Paul Corfield wrote

Singapore use touch out on alighting. It seems to work OK - the way
it works is that when you board the maximum fare is deducted from
the
card, then when touching out there is a refund if required.

True but the system is very clever in that it is linked to GPS and
switches off the exit validators until you are very close to the
stop.
This almost removes the possibility of people validating an exit and
not
actually getting off. The fact that interchange discounts are also
given
[...]
While I think the Singapore set up is neat I struggle to see how on
earth it could apply to the UK and particularly to London. We (LT)
did
spend a lot of time looking to see if we could reliably operate multi
fare pay as you go on buses with only entry validation or whether
exit
validation (passively or actively) was viable. The conclusion was
that
it couldn't work with the technology on offer at that time. It's
possible things have advanced but I remain skeptical about the odds
of
success.

As I have previously noted, putting the touch out devices outside at
bus stops rather than inside the bus doesn't require more advanced
anything. Thus in Tfl terms a surcharge would be added on entry, say on
all routes that go outside the zones. Boxes would be installed at all
bus stops on line of route which would refund the surcharge on touching
out if the stop is within the zones.


--
Mike D