InsaneJournal

King County Metro: Stored-Value Cards

As I sit here in the library, listening to the subtle music of the floor being abraded, I write. I have nothing better to do while waiting for 300MiB of data to copy to my bulla (rfriel knows what I'm talking about, so ask him).

Some people, most notably tsukiyomijapan, are always whining about how the Metro system should have stored-value cards so that users don't have to carry about change. I agree, but there are several technical problems with that sort of thing.

In order for a stored-value card to work, the value must be stored. (duh) Early stored-value systems (ATMs in the 1960s) kept the balance on the card and phoned in updates all at once, at night, because phone charges were expensive. Of course, this allows a user to just use a magcard thingy to change the number encoded on his card and take all the fare he wants.

Modern subway systems work well with stored-value systems because each ticket booth can be in constant (or at least frequent) contact with the central database, stopping this sort of fraud.

It is possible to have a copy of the master DB on each bus, in a computer under the driver's seat, but that would be a nightmare. Somebody would have to maintain around six thousand databases. Frightening indeed.

So somehow the buses must be in constant contact with the central database, and the cards can only know what their account numbers are. But how are they going to stay in contact? Packet radio sounds like the only option. You have six thousand buses, all competing for airspace. Sounds like more fun, but it would be doable and all that. (And what about reception problems? Metro goes all over the county, so they would need a very powerful transmitter on each bus, or else a massive repeater network.)

It would be very expensive, though. All the buses would have to be upgraded and a new system would have to be put in place. Worst of all, it would be very confusing for some buses to have the SV system and others not to, so it would have to be done all at once.