On Sun, 05 Nov 2006 17:08:03 GMT, David of Broadway wrote:

A Travelcard system with the capability to issue automatic ticket
extensions requires entry and exit swipes just as much as pure PAYG. A
regular commuter between Kenton and Central London can get away with a
Z1-2 Travelcard (£888 annually) rather than the proper Z1-4 Travelcard
(£1264 annually) -- a 30% savings -- in exchange for the risk of an
occasional £20 penalty charge on an inbound trip (but not on an outbound
trip).

(I would have used Harrow & Wealdstone as my example, for a more
dramatic 41% savings, but I'm not sure if Harrow & Wealdstone has gates,
while I know Kenton doesn't.)

I don't understand why PAYG abuse is such a problem while Travelcard
abuse is not.

I don't think that's the case. It's just that the solution to the PAYG
abuse can be implemented without penalising honest users[1], but the
same can't be said for Travelcards (in your example above, as far as
the system knows, they could have been travelling between Kilburn High
Road (which AFAIK doesn't have validators) and London, or Carpenders
Park (which is outside the zones) and London on a paper extension
ticket).

[1] At least in theory. But as many people who have used PAYG for any
length of time have found, it's only true most of the time.