Tri-Met's 251 cars a day

Those of us in the Portland area are familiar with the transit systems's buses and that commonly found claim on the side that "251 cars are at home because I'm on the road."
Well, as Steven Beaven in the Oregonian reports

It's not that the math is fuzzy. Rather, the labyrinthine calculations the transit agency used to arrive at 251 don't actually determine the number of cars left in driveways and garages each day. Instead, they estimate how many car trips are saved by each bus each weekday, based on the number of times riders board TriMet buses, not counting transfers.

Got that? So that 251 number includes each trip a single person might make in a day. If you use the bus to get downtown for work, and then back again in the afternoon, that counts as two trips. So the actual number of cars not on the road would be something like 175. But does that include trips to the store, before going to work? Or does it include stops for errands on the way home?
Just something to think about while you are behind a bus in rush hour traffic today.