This post is about situations in which the most important thing for transportation is reliability, more so than average speed or convenience. It’s inspired by two observations, separated by a number of years: one is my own about flying into or out of Boston, the other is from a New York Times article from yesterday describing a working-class subway rider’s experience.
My observation is that over the years, I’ve used Logan Airport a number of times, sometimes choosing to connect via public transportation, which always involves a bus as the airport is not on the rail network, and other times via taxi or pickup. My choice was always influenced by idiosyncratic factors – for example, which Boston subway line my destination is on, or whether I was visiting someone with a car and free time. However, over the last eight years, a consistent trend is that I am much more likely to use the bus arriving at the airport to the city than departing. I know my own reasoning for this: the bus between South Station and the airport is less reliable than a cab, so when in a crunch, I would take a train to South Station (often from Providence) and then hail a taxi to the airport.
The New York Times article is about a work commute, leading with the following story:
Maribel Burgos barely has time to change into her uniform before she has to clock in at the McDonald’s in Lower Manhattan where she works, even though she gives herself 90 minutes to commute from her home in East Harlem.
It does not take 90 minutes to get between East Harlem and Lower Manhattan on the subway. The subway takes around half an hour between 125th Street and Bowling Green, and passengers getting on at one of the local stations farther south can expect only a few minutes longer to commute with a cross-platform change at Grand Central. Taking walking and waiting time into account, the worst case is around an hour – on average. But the subway is not particularly reliable, and people who work somewhere where being five minutes late is a firing offense have to take generous margins of error.
When is reliability the most important?
What examples can we think of in which being late even by a little bit is unacceptable? Let us list some, starting with the two motivating examples above:
- Trips to the airport
- Work trips for highly regimented shift work
- Trips to school or to an external exam
- Work trips for safety-critical work such as surgery
- Trips to an intercity train station
In some of these cases, typically when the riders are of presumed higher social class, the system itself encourages flexibility by arranging matters so that a short delay is not catastrophic. At the airport, this involves recommendations for very early arrival, which seasoned travelers know how to ignore. At external exams, there are prior instructions of how to fill in test forms, de facto creating a margin of tolerance; schools generally do not do this and do mark down students who show up late. Doctors as far as I understand have shifts that do not begin immediately with a life-critical surgery.
But with that aside, we can come up with the following commonalities to these kinds of trips:
- They are trips to a destination, not back home from it
- They are trips to a fairly centralized and often relatively transit-oriented destination, such as a big workplace, with the exception of regimented shift work for retail (the original NY Times example), which pays so little nobody can afford to drive
- They are disproportionately not peak trips, either because they are not work trips at all, or because they are work trips for work that is explicitly not 9-to-5 office work
- They are disproportionately not CBD-bound trips
The first point means that it’s easy to miss this effect in mode choice, because people can definitely split choice between taxis and transit or between different transit modes, but usually not between cars and transit. The second means that driving is itself often unreliable, except for people who cannot afford to drive. The third means that these trips occur at a point in time in which frequency may not be very high, and the fourth means that these trips usually require transfers.
What does reliability mean?
Reliability overall means having low variance in door-to-door trip time. But for the purposes of this discussion, I want to stress again that trips to destinations that require unusual punctuality are likely to occur outside rush hour. Alas, “outside rush hour” does not mean low traffic, because midday and evening traffic in big cities is still quite bad – to take one New York example with shared lanes, the B35 steadily slows down in the first half of the day even after the morning peak is over and only speeds up to the 6 am timetable past 7 pm. Thus, there are twin problems: frequency, and traffic.
Traffic means the vagaries of surface traffic. Buses are generally inappropriate for travel that requires any measure of reliability, or else passengers have to use a large cushion. Everything about the mixed traffic bus is unreliable, from surface traffic to wait times, and bunching is endemic. Dedicated lanes improve things, but not by enough, and unreliable frequency remains a problem even on mostly segregated buses like the Silver Line to the airport in Boston.
Frequency is the harsher problem. The worker commuting from Harlem to Lower Manhattan is if anything lucky to have a straight-short one-seat ride on the 4 and 5 trains; most people who need to be on time or else are not traveling to city center and thus have to transfer. The value of an untimed transfer increases with frequency, and if every leg of the trip has routine 10-minute waits due to bunching or just low off-peak frequency guidelines, the trip gets intolerable, fast.
What’s the solution?
Bus redesigns are a big topic in the US right now, often pushed by Jarrett Walker; the latest news from Indianapolis is a resounding success, boasting 30% increase in ridership as a result of a redesign as well as other changes, including a rapid bus line. However, they only affect the issue of reliability on the margins, because they are not about reliability, but about making base frequency slightly better. New York is replete with buses and trains that run every 10-15 minutes all day, but with transfers, this is not enough. Remember that people who absolutely cannot be late need to assume they will just miss every vehicle on the trip, and maybe even wait a few minutes longer than the maximum advertised headway because of bunching.
Thus, improving reliability means a wider toolkit, including all of the following features:
- No shared lanes in busy areas, ever – keep the mixed traffic to low-traffic extremities of the city, like Manhattan Beach.
- Traffic signals should be designed to minimize bus travel time variance through conditional signal priority, focusing on speeding up buses that are running slow; in combination with the above point, the idea of giving a late bus with 40 passengers the same priority at an intersection as a single-occupant car should go the way of the dodo and divine rights of kings.
- Off-peak frequency on buses and trains needs to be in the 5-8 minute range at worst.
- Cross-platform transfers on the subway need to be timed at key transfer points, as Berlin manages routinely at Mehringdamm when it’s late and trains run every 10 minutes (not so much when they run every 5); in New York it should be a priority to deinterline and schedule a 4-way timed pulse at 53rd/7th.
- Branch scheduling should be designed around regular gaps, rather than crowding guidelines – variation between 100% and 130% of seats occupied is less important to the worker who will be fired if late than variation between waiting 4 and waiting 8 minutes for a train.
- Suburban transit should run on regular clockface schedules every 30, 20, or 15 minutes, with all transfers timed, including with fare-integrated commuter trains.