That is, in fact, a public service announcement, not Prostate-Specific Antigen. Although I'm moving towards the age where I'll need to get that checked.
Anyway: this announcement is for the website NextBus--they have installed GPS units in buses, so that they can post predicted wait times for a bus on a given line (as per their "about" page):
NextBus uses satellite technology and advanced computer modeling to track vehicles on their routes. Each vehicle is fitted with a satellite tracking system.
Taking into account the actual position of the buses, their intended stops, and the typical traffic patterns, NextBus can estimate vehicle arrivals with a high degree of accuracy. This estimate is updated constantly.
I remember seeing this system out a few years ago on MUNI buses out in the Bay Area... wow, so not dumb! I was pretty psyched to find out that it had come to the MBTA.
So how does it work? Um... mostly well. I've been using it on the 77 bus, and the Silver Line at the airport, with my BlackBerry web browser. You can hit update, and it corrects timing. But there are occasional snags... times bouncing back and forth, or going from "3 minutes" to "5 minutes" to "arriving" (a bit like the xkcd strip, "the author of the Windows file copy dialog visits some friends").
But hey... definitely much better than just sitting around feeling frustrated. Also, useful for figuring out, "Do I have a stupid-long wait, that I can use to run into CVS and pick something up?" Although I've been biking to work a lot lately (given the great weather), I'm glad that this service will be around come winter, when I'll be bus-bound a lot more.