Why does the Massachusetts Bay Transit Authority have schedules? I notice on my “Axonometric Projections of Harvard Square, Central Cambridge, Central Square, Cambridge Center & M.I.T. Areas” map — if it isn’t redundant to say “map” after saying “axonometric projections” — that someone has humorously given creator Tom Kane frequencies for buses and the T.

If you take the red line toward either Ashmont or Braintree, it says, you can expect a T to come along every eight minutes during rush hour, every 12 minutes during other weekday times, every 13 minutes on Saturday and every 15 minutes on Sunday.

Very interesting, and completely meaningless. The T can come much more frequently during any time of the day, or two can come almost immediately, or three, and then others can lurk elsewhere for 20 or 25 minutes at a time. Every once in a while the drivers announce, no doubt holding back chuckles, that they must keep a train frozen on the tracks for a couple of minutes for “a schedule adjustment.” Schedule? If the T is on a schedule, why isn’t the schedule posted? Why aren’t even these map figures posted somewhere? Why must one spend $4.95 at the travel bookstore in Harvard Square to discover them?

Bus times are posted, but it’s a mystery why, as the MBTA can’t keep to them. Bus consistency is about equal to T consistency, but the disappointment is more acute because there is a schedule,  painstakingly broken down by minute of arrival and departure at stops along a route — passengers can consult it, MBTA officials create it, and it bears little relation to reality.

The map is dated 1998, so the frequencies given for the T’s red line may have been abandoned. If so, the bus schedules should be abandoned, too, perhaps in favor of a T-style frequency chart, perhaps in favor of big signs saying merely that “A bus is coming.” The advantage is that, instead of breaking promises dozens of times a day, the phrase promises very little; a bus is coming, even if you’ve missed the last one for the day and you must wait six or so hours for the next day’s first bus to arrive.

Finally, I must mention how disconcerting it is to have lived here for so many years and still be put off and confused by the map referring to “Central Cambridge, Central Square and Cambridge Center” as three different things.