Revised in response to early comments.

Are you sure you know which of your transportation options is fastest? It depends on how you think about travel time.

A recent Boston Magazine article about the private bus service Bridj featured typical "race" between two transit modes: the MBTA subway and Bridj, which provices luxury buses on fixed routes and schedules running only at times of peak commute demand. The newspaper sent someone by each path at the same time. The outcome of the race is supposed to be decisive:

Why is this not a fair race? Well, it depends on when you start. From the article:

The MBTA passenger arrived last, [sic] even though she had a head start and boarded the train six minutes prior to Bridj’s departure.

Why a six minute headstart? Why not 10 or 20? What headstart would be appropriate? The headstart is your cue that there's something wrong with this methodology.

What's really happening here is that a service that is available all the time — the subway — is being compared to one that's only available at a few special times — Bridj's specialized commuter buses. Any "travel time race," with any headstart, is going to miss the real point of this comparison.

The notion of travel time seems so self-explanatory that most people miss how deeply misleading it is in discussing transit. The imagined user is someone who happens to be going at the ideal moment for the preferred mode to succeed. We talk about travel time this way because it's how motorists, cyclists, and pedestrians experience it: as something that begins at the moment you want to go.

But that's not how transit travel time works, so the comparision implied by the term "transit travel time" is often a false one.

When I teach transit planning and rhetoric, I encourage people to think of a weighted sense of travel time that includes average wait time, or more generally the difference between when you wanted to go and when you went. A bus that's 10 minutes faster is of no use if gets you somewhere 30 minutes before you needed to be there [an 8:00 AM class or meeting, for example] because that's the only time it ran.

Purveyors of low-frequency transit services, such as classical North American commuter rail, do this as well, bragging about how fast you can get from A to B without mentioning that this travel time is available only once a day.

Unless you are sure that you will absolutely always travel at the same time each day, transit travel time figures have to be viewed with skepticism. Whenever you hear about travel time, ask about frequency!