A great piece by Michael Perkins in Greater Greater Washington highlights a perennial problem with on-time performance measures for urban buses. He cites the policy of the Washington area transit agency, WMATA, which says that a bus is considered on-time if it’s no more than two minutes early and no more than seven minutes late. Perkins explains, with diagrams, that under this policy you could wait 19 minutes for a bus that supposedly ran every ten minutes, and yet the bus (and the one 19 minutes in front of it) would both be considered on-time.
(By the way, the WMATA standard sounds lax to me, though I haven’t done a survey. Few agencies I’ve worked with accept anything more than five minutes late, or one minute early.)