[coopr]

And yet it often is. And, even when it isn't, knowing how much to pay can be hard.

The ideal system is one in which you use the "universal transit payment card" desired in the article to get on the transit system, then use it again to get off (if the transit system requires it - no all do), without having to give even a single thought to how much money you are spending.

When you have different payment cards/passes/tickets in different systems, you have to figure out not only how to buy those tickets, but how much to buy (so that you don't depart the area having vastly overbought - or so that you don't run out of loaded fare value before you end your trip).