Given today's economic issues, are bus systems capable of rapid scaling?
I would expand that to say any public transport in the UK.
Infrastructure improvements take far too long to deliver, and the real cost of delivery has risen over the last 50 years to the point where schemes are just not going to find funding.
Any scheme, travel card etc. is going to have to find a way of not overloading the existing system. Where trains or buses are already fully loaded there is no point in trying to encourage more useage.
I think that before any sort of card is introduced the whole fares system needs an overhaul. In the current climate it would have to be revenue neutral. But given that 3rd party websites can offer cheaper fares using split tickets, why cant the fare system be revised to deliver those fares to everyone.
I would get down to 3 simple fare categories for most journeys, Anytime, Offpeak, Advance, no TOC specific journeys, the Advance category will cover those fares. If we keep advance and offpeak returns then they are twice the price of a single. The fare offered in a category (Anytime, Offpeak, Advance) will always be the best fare available between the two points with no savings available by splitting tickets.
Having sorted out the fares structure the next step is to try and integrate the various modes of public transport. If you want to get more people on public transport then mixed mode journeys have to be integrated, with through ticketing and guaranteed completion of the journey in the event of disruption. This would probably be best done on area by area basis, a local example would be Hull, where every bus route in the area is integrated for ticketing. It would have to be a rolling process, and there could be resistance from bus operators, but as most of them are I think in the receipt of subsidies from one source or another, the subsidy can be both the stick and the carrot to get them into an integrated system.
Once integration is acomplished for an area then a local travel card could be introduced. As adjacent areas are brought in that option could be extended to cover more zones on payment of a higher subscription. Eventually all but the most remote areas would be covered.
This would take time and vision to deliver, and to be honest I dont see anyone on the political landscape with the drive to see it through, in any of the main parties.