"NextBus uses Global Positioning System (GPS) satellites to provide real-time transit arrival information – not just schedules – to passengers and managers of public transit."
Here’s how it works. The system constantly updates its vehicle arrival estimates.
03/03/02 update: thanks to Jason for his feedback:
"The Web part of NextBus works pretty well. If I’m going somewhere on the MUNI, I pull it up and hop outside when they say the train is coming. It can be a little sporadic on the weekends, but it’s fine otherwise. I’ve never used their wireless stuff, so I don’t know how well that works."