Usability 
NextBus is an Alameda, California supplier of real time passenger information technology to the transit industry. The technology delivers frequently updated estimates of the arrival time of the next transit vehicle at a given stop. Delivery is by electronic signs, voice response systems and the Internet including the mobile web. NextBus is currently operational on 45 transit systems in the US and Canada.
NextBus' real time transit information can be accessed at http://www.nextbus.com. If a wml capable browser is detected, a wml version of the site is shown, otherwise a mobile or desktop web page appears.The browser detection isn't perfect, Opera Mini is detected as a desktop browser. The direct URL to the mobile version is nxtb.us/mini/index.webui The content of both mobile versions appears to be essentially identical. They work well although the UI could be improved; there is too much drill down required to the actual real-time information. First you pick the State, then the transit agency, then the transit line, then the direction and finally the stop that you want times for. Once you find your stop, bookmark it so you can get back to it without repeating the drilldown. I like that NextBus tells you in how many minutes the next bus will arrive, along with the current time. Many other systems tell you at what time the next vehicle will arrive, which assumes that your watch is synchronized with the information system's time base. Several of the other systems don't even tell you the current time. I realize that all mobile phones have a real time clock, but on feature phones checking it often requires getting out of the browser. Plus there is no guarantee that the phone and tracking system's clocks are in sync.
I find that NextBus works very well for my commute on the MUNI in San Francisco. I have a choice of taking a bus that stops two blocks from my house or a streetcar three blocks away. The streetcar is covered by NextBus but the bus is not. Both lines supposedly run every twenty minutes but in reality are rarely on time. As I leave my house, I check the streetcar on NextBus and if there is one coming in between 5 and 15 minutes I head down to the streetcar stop and catch it, otherwise I take my chances with the bus. It is very reassuring to know exactly when the streetcar will arrive. The accuracy of NextBus is uncanny. When NextBus says the streetcar will arrive in one minute, I can always see it down the tracks a block or two away.
NextBus tracks all of San Francisco MUNI lines. For some of the othe agencies only a subset of lines are covered by NextBus. I suspect that cost considerations are to blame for the less than full coverage on some systems.. Each vehicle needs a GPS receiver and radio transmitter and of course there is the expense of the software and web hosting. An approximate idea of the cost can be gleaned from this public document (pdf) which shows that it will cost AC-Transit, another San Francisco Bay Area agency about $1 million for equipment, implementation and seven years of operating expenses for a NextBus system covering 199 buses on 25 routes including 44 electronic signs along with mobile and web access.