Why changing trains is good for you

Ideal. Image: Tokyo Transit authority.

To complete your trip in a world-class transit system, you may have to make a connection, or "transfer" as Americans say. That is, you may have to get off one transit vehicle and onto another. You probably don't like doing this, but if you demand no-transfer service, as many people doyou may actually be demanding a mediocre network for your city.

There are several reasons for this, but let's start with the most selfish one: your travel time. 

Imagine a simple city that has three primary residential areas, along the top in this diagram, and three primary activities of employment or activity, along the bottom.

In designing a network for this city, the first impulse is to try to run direct service from each residential area to each activity centre. If we have three of each, this yields a network of nine transit lines:

Suppose that we can afford to run each line every 30 minutes. Let’s call this the Direct Service Option.

Now consider another way of serving this simple city for the same cost. Instead of running a direct line between every residential area and every activity centre, we run a direct line from each residential area to one activity centre, but we make sure that all the resulting lines connect with each other at a strategic point. 

Now we have three lines instead of nine, so we can run each line three times as often at the same total cost as the Direct Service option. So instead of service every 30 minutes, we have service every 10 minutes. Let's call this the Connective Option.

Asking people to "transfer" is politically unpopular, so the Direct Service option is the politically safe solution, but if we want to maximize mobility with our fixed budget, we should prefer the Connective option. Consider how long at typical trip takes in each scenario, from the standpoint of a person whose needs to leave or arrive at a particular time. 

Let’s arbitrarily look at trips from Residential Area 1 to Activity Area 2. For simplicity, let's also assume that all the lines, in all the scenarios, are 20 minutes long.

In the Direct Service scenario, a service runs directly from Residential Area 1 to Activity Area 2. It runs every 30 minutes, so on average, the waiting time is 15 minutes. Once we’re on board, the travel time is 20 minutes. So the average trip time is:

Wait 15 minutes 

+  Ride 20 minutes

35 Minutes.

Now look at the Connective Option. We leave Residential Area 1 on its only line, which runs every 10 minutes, so our average wait is 5 minutes. We ride to the connection point and get off. Since this point is halfway between the residential areas and the activity centres, the travel time to it is 10 minutes. Now we get off and wait for the service to Activity Area 2. It also runs every 10 minutes, so our average wait time is 5 minutes. Finally, our ride from the connection point to Activity Area 2 is 10 minutes. So our average trip time is:

Wait 5 minutes

+ Ride 10 minutes

+ Wait 5 minutes

+ Ride 10 minutes

= 30 minutes.     

The Connective Network is faster, even though it imposes a connection, because of the much higher frequencies that it can offer for the same total budget.

As cities grow, the travel time advantages of the Connective Network only increase. For example, suppose that instead of three residential areas and three activity centres, we had six of each. In this case, the direct-service network would have 36 routes, while the connective network would have only six. You can run the numbers yourself, but the answer is that the Direct Service network still takes 35 minutes, while the Connective network is down to only 25 minutes, due to the added frequency.

Lets anticipate a couple of objections to this thinking.

The Modeller’s Objection

If we were actually using travel time as a means of estimating ridership, we would have to consider the widespread view, built into most ridership models, that connections impose a “transfer penalty” in addition to the actual time it takes. These penalties assume that even though people say they want the fastest possible trip, they'll actually prefer a slower trip if it saves them the trouble of getting out of their seat partway through their journey.

In the above example, for example, a model might assume that although the average trip in the Connective option is faster, the Direct Service option would give us higher ridership, because the Connective option imposes the inconvenience of the connection. The modeller might say that this inconvenience is the equivalent of 10 minutes of travel time, so that the Connective option will really attract ridership as though the trip took 40 minutes instead of 30. This common modelling approach assumes that the inconvenience of transferring is something different to, and separable from, the time that the transfer takes. 

There is considerable documentation behind the addition of this kind of factor, but the unpleasantness of the connection experience depends on many details of how the connection works. If two buses or trains arrive on opposite sides of a platform, facing one another five meters apart, with their doors open at the same time, walking out of one and into the other is a pretty low level of inconvenience for most passengers. If the connection involves getting off a bus, crossing a busy street, and waiting for another bus not knowing when it will arrive, the inconvenience is much greater. 

So the configuration of the connection matters. Transfer penalties are based on a crude averaging of many different types of connection experience, so good interchanges will reduce these penalties. Modelling assumptions about a "transfer penalty" (as distinct from the time the connection takes) deserved to be scrutinized: what kind of connection experience was used to calibrate the model? 

The 9-to-5 Commuter's Objection 

People who commute regularly might well object to the way I've inferred average waiting times from frequencies. After all, if a particular airline route has one flight a day, that doesn’t mean we have spend half the day waiting for it. We go on with our lives and work, and go catch the flight whenever it is leaving. Many people do treat commuter service schedules in this way. Even if the bus runs every 30 minutes, they’ll just do other things until it’s due, and then go out to catch it. 

However, the average wait is still a valid way of capturing the inconvenience of low-frequency services. For example, if you need to be at work at 8:00 and your bus is half-hourly, you may have to take a bus that gets you to work at 7:35. This means that every morning, you’ll have 25 minutes at your destination before work starts, time you’d probably rather have spent in bed. You may figure out how to make use of this time, but it’s still time you must spend somewhere other than where you want to be. 

Note too that for simplicity we have presented this example in terms of commutes to work, but of course, a good public transport system serves many kinds of trips happening all day. You may figure out how to make use of a predictable 25 minute delay at the beginning of your work day, but it’s much harder to deal with unpredictable 25 minute gaps in the many trips that you need to make in the course of the day, such as while taking a lunch break or running errands that involve many destinations. So all-day frequency still matters.

Other Advantages of Connective Networks

There's a few further factors which argue for Connective networks over Direct Service networks:

  • Average travel time is better than the worst-case time calculated above. In the Direct Service network, everybody’s trip takes 35 minutes. In the Connective network, two-thirds of the market has a 30-minute trip, but one-third of the market (those still served by a direct route) has an even faster trip. 
  • The Connective network is made of more frequent services. Frequency makes connections faster but it also stimulates ridership directly, especially when we consider the needs of people who have to make several trips in a day, or who want to travel spontaneously, and who therefore need to know that service is there whenever they need it.
  • The Connective network is simpler. A network of three frequent lines is much easier to remember than a network of nine infrequent ones. Marketing frequent lines as a Frequent Network can enhance the ridership benefits of this simplicity. 

Most transit networks start out as Direct Service networks with relatively little focus on connections, but as the city grows bigger and more complex, connections become more important. In most cases, though, there’s a transition from a Direct Service network to a Connective one, a transition that often requires severing direct links that people are used to in order to create a connection-based structure of frequent service that is more broadly useful and legible. 

Helping agencies through this hard step is one of my specialties as a consultant, and while there's usually a moment in the process where the resistance seems overwhelming, the agencies I've worked with are almost all glad that they broke through this resistance, because the result was a network that was much more frequent, and therefore more relevant to the life of the city.

Images courtesy of the author.

Jarrett Walker is an international consultant in public transit network design and policy, based in Portland, Oregon. He is also the author of “Human Transit: How clearer thinking about public transit can enrich our communities and our lives".

This article was originally written for his blog, and is reposted here with permission.



17 things the proposed “Tulip” skyscraper that London mayor Sadiq Khan just scrapped definitely resembled

Artist's impression. See if you can guess which one The Tulip is. Image: Foster + Partners.

Sadiq Khan has scrapped plans to build a massive glass thing in the City of London, on the grounds it would knacker London’s skyline. The “Tulip” would have been a narrow, 300m skyscraper, designed by Norman Foster’s Foster & Partners, with a viewing platform at the top. Following the mayor’s intervention, it now won’t be anything of the sort.

This may be no bad thing. For one thing, a lot of very important and clever people have been noisily unconvinced by the design. Take this statement from Duncan Wilson, the chief executive of Historic England, from earlier this year: “This building, a lift shaft with a bulge on top, would damage the very thing its developers claim they will deliver – tourism and views of London’s extraordinary heritage.”

More to the point, the design was just bloody silly. Here are some other things that, if it had been built, the Tulip would definitely have looked like.

1. A matchstick.

2. A drumstick.

3. A cotton ear bud.

4. A mystical staff, of the sort that might be wielded by Gandalf the Grey.

5. A giant spring onion.

6. A can of deodorant, from one of the brands whose cans are seemingly deliberately designed in such a way so as to remind male shoppers of the fact that they have a penis.

7. A device for unblocking a drain.

8. One of those lights that’s meant to resemble a candle.

9. A swab stick, of the sort sometimes used at sexual health clinics, in close proximity to somebody’s penis.

10.  A nearly finished lollipop.

11. Something a child would make from a pipe cleaner in art class, which you then have to pretend to be impressed by and keep on show for the next six months.

12. An arcology, of the sort seen in classic video game SimCity 2000.

13. Something you would order online and then pray will arrive in unmarked packaging.

14. The part of the male anatomy that the thing you are ordering online is meant to be a more impressive replica of.

15. A building that appears on the London skyline in the Star Trek franchise, in an attempt to communicate that we are looking at the FUTURE.

14a. Sorry, the one before last was a bit vague. What I actually meant was: a penis.

16. A long thin tube with a confusing bulbous bit on the end.

17. A stamen. Which, for avoidance of doubt, is a plant’s penis.

One thing it definitely does not resemble:

A sodding tulip.

Anyway, it’s bad, and it’s good the mayor has blocked it.

That’s it, that’s the take.

(Thanks to Anoosh Chakelian, Jasper Jackson, Patrick Maguire for helping me get to 17.)

Jonn Elledge is editor of CityMetric and the assistant editor of the New Statesman. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.

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