Frequency is Freedom. What is Span?

I’ve been revisiting Jarrett Walker’s book Human Transit¹. Reading it crystalized the disjointed thoughts that I had as a transit rider into opinions on how public transit networks should operate. It is one of the most important books I have read, and I suspect that is true for many people with a stake in public transit.

I’m revisiting it by way of processing the response to my cost-neutral redesign of King County Metro’s network, which overlaid Seattle with 24/7 frequent service . It appeared in Seattle Transit Blog, where comments on it were more uniformly negative than I anticipated. These commenters are not fools. They recognized that widespread frequent overnight service would involve tradeoffs, like more crowding at rush hours, and fewer one-seat rides to population and activity centers like downtown and the University District. They insisted that riders would not favor this exchange.

This reaction seemed rooted in skepticism of the value of an extended span of service: the interval between a route’s first and last trip of the day. In the proposal, all routes have either eight-, 10-, 12-, or 15-minute headways. For many routes in Seattle, these are not massive improvements over the present midday frequency, if they are improvements at all. The main difference in this proposal is that routes maintain these high frequencies 24 hours per day, seven days per week, even as demand for travel declines at night. A common theme among comments was that the investment in all-day service would be better used boosting frequency over a more conventional span.

Given how influential Human Transit is, I thought it might offer some insight into this reaction. While the book extensively discusses frequency, it has little to say about span. There are only three references to it in the index and, in each of them, the conversation quickly moves toward frequency. Span and frequency are never discussed as acting in opposition, even though they are competing outlets for investment in a transit route. The book does discuss the related tradeoff between all-day and heavily peaked service, but it does not generally weigh in on how early a route’s service should start and how late it should run. Walker does note that a route’s last trip of the day tends to see fewer riders because they want a fallback plan that the last trip cannot provide. The idea of there not being a last trip is never considered, even as a hypothetical.

Human Transit’s lack of emphasis on span extends to its discussion of the ridership-coverage tradeoff that transit agencies make. Walker illustrates the tradeoff using a fictional municipality with a fixed number of buses available. In this city, population and desirable destinations are mostly concentrated along two intersecting main roads. Fewer homes and opportunities exist along a set of secondary roads that wind through other parts of the city. In the ridership network, there are two transit routes, one per main road. The coverage network creates nine transit routes, providing transit service on every street, but the individual routes are served with fewer vehicles.

The variation in coverage is apparent, but the connection to ridership is less explicit. Assume that the buses use no fuel and don’t require drivers, so there’s no downside to running them all the time. Consolidating buses on to fewer transit routes results in a greater frequency of trips. Frequent service allows time-sensitive and spontaneous trips to be made; as the book emphatically and memorably states, “frequency is freedom”. This freedom, and that many people are originating at or intending to reach destinations along these roads, means the service will be well used. In contrast, the lower frequency of routes on the coverage network limits how often transit will be useful for making trips.

The necessity of paid human drivers complicates the ridership-coverage tradeoff. While the example uses buses as a unit, the book explains elsewhere that transit agencies generally hit the limits of their labor budget before running out of vehicles. The resource actually distributed among routes is a budget of service time. If an agency changes its service allocation from more routes to fewer, it may not elect to improve the remaining routes’ frequencies. It may choose instead to improve their span by adding trips before or after the current span of service. Unfortunately, the way that the example is constructed creates an association only between frequency and ridership.

Like high frequency, though, long span enables spontaneous and unanticipated trips. This broadens the variety of uses for which transit can be trusted, increasing ridership. If “frequency is freedom”, then span is too.

What if the rallying cry were instead “frequent span is freedom”? While this is more comprehensive, it’s worse in every other way. The value of “frequency is freedom” is a lack of ambiguity: make transit come more often and people are more free. “Frequent span” mashes two concepts together. If additional trips can be added, should they increase frequency within the current span of service or lengthen it with its current frequency? Adding span to the equation complicates something that has been effective because of its simplicity. A real solution must be more thoughtful than stapling span onto “frequency is freedom”.

Human Transit has brought about undeniable progress in improving conversations about the frequency of transit service. It has encouraged planners to view frequency not just as a capacity management tool, but to recognize that increasing it, even when crowding doesn’t strictly demand it, is a way to make transit more trusted and ridden. Span remains approached from a capacity perspective; the belief persists that the number of people traveling at night is insufficient to run any trips, much less frequent service. Needs arise after dark, though, and public transit can’t meet them. This degrades trust in the system the same way that infrequent service does. What will do for span what Human Transit did for frequency, and bring about clearer thinking on it?