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.



What's actually in the UK government’s bailout package for Transport for London?

Wood Green Underground station, north London. Image: Getty.

On 14 May, hours before London’s transport authority ran out of money, the British government agreed to a financial rescue package. Many details of that bailout – its size, the fact it was roughly two-thirds cash and one-third loan, many conditions attached – have been known about for weeks. 

But the information was filtered through spokespeople, because the exact terms of the deal had not been published. This was clearly a source of frustration for London’s mayor Sadiq Khan, who stood to take the political heat for some of the ensuing cuts (to free travel for the old or young, say), but had no way of backing up his contention that the British government made him do it.

That changed Tuesday when Transport for London published this month's board papers, which include a copy of the letter in which transport secretary Grant Shapps sets out the exact terms of the bailout deal. You can read the whole thing here, if you’re so minded, but here are the three big things revealed in the new disclosure.

Firstly, there’s some flexibility in the size of the deal. The bailout was reported to be worth £1.6 billion, significantly less than the £1.9 billion that TfL wanted. In his letter, Shapps spells it out: “To the extent that the actual funding shortfall is greater or lesser than £1.6bn then the amount of Extraordinary Grant and TfL borrowing will increase pro rata, up to a maximum of £1.9bn in aggregate or reduce pro rata accordingly”. 

To put that in English, London’s transport network will not be grinding to a halt because the government didn’t believe TfL about how much money it would need. Up to a point, the money will be available without further negotiations.

The second big takeaway from these board papers is that negotiations will be going on anyway. This bail out is meant to keep TfL rolling until 17 October; but because the agency gets around three-quarters of its revenues from fares, and because the pandemic means fares are likely to be depressed for the foreseeable future, it’s not clear what is meant to happen after that. Social distancing, the board papers note, means that the network will only be able to handle 13 to 20% of normal passenger numbers, even when every service is running.

Shapps’ letter doesn’t answer this question, but it does at least give a sense of when an answer may be forthcoming. It promises “an immediate and broad ranging government-led review of TfL’s future financial position and future financial structure”, which will publish detailed recommendations by the end of August. That will take in fares, operating efficiencies, capital expenditure, “the current fiscal devolution arrangements” – basically, everything. 

The third thing we leaned from that letter is that, to the first approximation, every change to London’s transport policy that is now being rushed through was an explicit condition of this deal. Segregated cycle lanes, pavement extensions and road closures? All in there. So are the suspension of free travel for people under 18, or free peak-hours travel for those over 60. So are increases in the level of the congestion charge.

Many of these changes may be unpopular, but we now know they are not being embraced by London’s mayor entirely on their own merit: They’re being pushed by the Department of Transport as a condition of receiving the bailout. No wonder Khan was miffed that the latter hadn’t been published.

Jonn Elledge was founding editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.