Generally speaking, late-night transit usage can be divided into four market segments; employment, recreation, shelter-seekers, and non-users.
In general, not very many shift changes occur between Midnight and 6 am, with the notable exceptions of shift ends in the hospitality, restaurant, and transportation industries until about 2-3 am, and shift starts in transportation and related service industries beginning at about 4 am. Absent any other noticeable industry influences (i.e. one-industry towns), any shift patterns not listed above are not common enough to be the basis of economical transit planning.
Recreational customers are essentially identical to hospitality and service employees in their trip patterns, but leave slightly earlier (the employees have to stay to clean up after their customers go home). This market segment is more likely travelling on Friday and Saturday nights (early morning Saturday and Sunday), as well as on the eves of certain holidays, and conversely are less likely to travel on nights preceding a work or school day. The above two groups share the same common travel characteristic that they are moving away from destinations in the very early hours, with that flow decreasing as a flow of customers towards destinations increases closer to daytime.
In many cases, all-night service attracts people in seek of shelter due to homelessness, domestic abuse, etc. These customers tend to favor long, uninterrupted round-trip runs where they are able to sleep most effectively, essentially turning the bus into a roving shelter. Route 22 in San José has been referred to as “Hotel 22” due to the clientele; customers are riding because the bus is a shelter, not because the bus is transportation (for that specific trip). As heartbreaking it is to see the pictures of the 10-year-old girl slouched over bus seats, as well as read the stories of the various people in seek of shelter, and as important as addressing homelessness is as a public policy initiative, the practical reality is that transit firms are not, nor should not, be in the business of combating broader social issues. Transit firms are in no position to offer the services homeless or temporarily dislocated people need, and from a public policy perspective should make no effort to present any image in the collective public mindset that they are a good shelter resource.
While technically not customers now, the fourth market segment of note are people who would use transit service for other purposes. Most people are in bed, or at least home, between 1am-5am; the bus line could run every 5 minutes, once an hour, or not at all and it would not affect travel habits of the majority; if a tree falls in the woods and nobody is around to hear it…. For people with no car availability who do need to suddenly travel at night, inelastic but rare demand for immediate night travel is best met with taxicabs and similar on-demand services.
When the above is taken into account the “symbolic” service offered by the MBTA on a 15-20 minute headway, but only until 3 am is likely superior to service every 30-60 minutes but at all times. More potential customers are in a place to use frequent service to return home than infrequent service offered at times they have no need to travel.
(Note: Copied nearly verbatim from my comment on the Amateur Planner’s post on the MBTA’s late-night service trial.)