Speed vs coverage: How do metro systems decide how to space their stops?

The Paris Metro: quite close to another station, this. Image: Getty.

The Paris Metro averages a stop every 600m. The Moscow Metro averages a stop every 1.7km. Most of the world's largest systems are in between, several clustering in the 1.2-1.3 km range, including the London Underground, the Tokyo subway, and the Mexico City Metro.

But why is this? How come metro builders in some cities chose to build stations three times as far apart as in others? And what about those cities that have no metro system, but are building one, such as Tel Aviv or Sydney? What should they do?

The basic tradeoff here is between speed and coverage. Wider stop spacing means fewer locations have a metro station, but the speed between the stations is higher. The Moscow Metro averages 41 km/h, while the Paris Metro only averages about 25km/h. Other systems are intermediate: in Tokyo the average speed is about 30km/h; in London 33km/h.

There are other factors determining average speed, so that newer lines are often fast for their stop spacing. But each additional station adds about 40-60 seconds of travel time, depending on top speed, track quality, and train acceleration capabilities. The tradeoff, then, is the question: are more stations worth the extra travel time?

Each metro-building tradition answers this question differently. In cities where the metro extends deep into suburbia, stop spacing is wide; Paris built the RER as a separate system, with express stop pattern, because the Metro was too slow to effectively serve the suburbs.

Moreover, different countries make different decisions based purely on tradition. Under Parisian influence, the Montreal and Lyon Metros have short stop spacing; under Moscow's influence the metro systems in the former Communist Bloc, from Eastern Europe to China and North Korea, usually average more than 1.5 km between stations. With neither influence, cities in developing countries that build new metros, such as in South Asia, seem to use the same stop spacing as London or Tokyo.

But there is more to the stop spacing decision than the speed versus coverage tradeoff. Large cities, which expect to build many metro lines, need to plan how those lines will intersect in their cores. The San Francisco urbanist Brian Stokle wrote about the related subject of line spacing: how cities space parallel metro lines in their central business districts. Using American examples, Stokle argues that the typical space for parallel lines is 500-700 meters; this also appears to be the average in Paris and in central London.

The upshot is that if two lines are parallel, spaced about half a kilometer apart, then a line that intersects them orthogonally had better have two stops half a kilometer apart, for transfers. For example, in the diagram below the red and blue lines are roughly parallel, and the black line is orthogonal to them.

This looks familiar. Image: author provided.

Metro planners aim to provide a transfer station at the intersection of every pair of lines. In practice, because most metro systems have denser line spacing than stop spacing, this is not always feasible. Metro systems that feed geographically small central business districts, such as central London or central Tokyo, end up with multiple missed connections; New York, where the subway was built by three separate companies, has more than twenty missed connections. But usually, there is only a small handful of missed connections, often just one or two.

A separate question is that of express lines. In New York, five of the nine subway trunk lines have four tracks, with local and express trains; in Seoul, Line 1 has four tracks as well. Thanks to the express lines, New York maintains very narrow stop spacing on the local lines.


But a more common situation is one in which every metro line has two tracks, with all trains making all stops, on which some lines are more express than others. In Paris, the RER A was built as an express version of Metro Line 1, and, decades later, Metro Line 14 was built with longer stop spacing as well, to relieve the central segment of the RER A.

This situation leads to missed connections. The RER A tries to make connections when it can, but still crosses a few lines without a transfer, or else it would be hardly any faster than Line 1.

London's equivalent, Crossrail, does the same: it misses some connections to north-south lines, because if it didn't, it wouldn't be faster than the Central line, simply because line spacing in Central London is so dense. Within the Paris Metro, excluding the RER, there are three missed connections, two involving Line 14; an under-construction extension of Line 14 misses yet another connection. In Asia, several cities, including Hong Kong, Beijing, and Delhi have express lines to the airport, with missed connections in every case.

But it's easier to build networks with long stop spacing in newer cities, purely because of how their business districts are laid out. In old industrialised cities like London, Paris, New York, and even Tokyo, there is a dominant CBD, a few square kilometers in area, and most metro lines enter it. In all of these cities, the CBDs for the most part predate the metro system.

In newer cities in developing countries, the CBDs look different, with multiple centers, sometimes purpose-built. This leads to longer line spacing, matching the wide stop spacing. On same-scale maps of their networks, Paris, London, and Tokyo all look like hard-to-follow blobs in their centers, whereas Chinese cities, especially Beijing, still look clear. In Beijing, the only missed connection today involves the airport express line.

The most ideal metro network looks radial, with a circular line or two. Every pair of radial lines should intersect, once, with a transfer station, and every radial should intersect every circle twice, again with transfers. Ideally interchange stations should only involve two lines at a time, to avoid clogging the most popular locations. The diagram above is a good example of a coherent network with three lines. Unfortunately, the interaction of line spacing and stop spacing makes the ideal network difficult to construct. It's also unlikely that the street network is perfectly aligned for this; for example, cities with street grids, like Beijing or Philadelphia, can't easily build lines diagonally to the grid.

 

The ideal network? At least, if you ignore the chaos of that central station. Image: CityMetric.

This means that the only way to guarantee easy connections between metro lines in most large cities is to build very short stop spacing, as in Paris. Unfortunately, this imposes a sharp limit on train speed - and it's precisely the largest cities that have the most need for speed, since their suburbs usually stretch farther out of city center than those of smaller cities.

Metro construction is full of compromises. Cities that are building new systems, especially in the developed world, are likely to have so much sprawl, from decades of growing without a metro, that they need long stop spacing to serve the suburbs effectively. But they also are likely to have an organic central business district with many close-in dense neighborhoods, which would benefit from short stop spacing; they also have everywhere-to-everywhere commutes, as all modern cities do, which makes good interchanges between lines a must. Something has to give, and each city needs to figure out how, in its particular situation, to choose the optimal point in the speed-coverage tradeoff.

 
 
 
 

Older people need better homes – but then, so does everybody else

Colne, Lancashire. Image: Getty.

Towards the end of last year, I started as an associate director at the Centre for Ageing Better, working particularly on our goal around safe and accessible homes. Before I arrived, Ageing Better had established some ambitious goals for this work: by 2030, we want the number of homes classed as decent to increase by a million, and by the same date to ensure that at least half of all new homes are built to be fully accessible.

We’ve all heard the statistics about the huge growth in the number of households headed by someone over 65, and the exponential growth in the number of households of people over 85. Frustratingly, this is often presented as a problem to be solved rather than a major success story of post war social and health policy. Older people, like everyone else, have ambitions for the future, opportunities to make a full contribution to their communities and to continue to work in fulfilling jobs.

It is also essential that older people, again like everyone else, should live in decent and accessible homes. In the last 50 years we have made real progress in improving the quality of our homes, but we still have a lot to do. Our new research shows that over 4 million homes across England fail to meet the government’s basic standards of decency. And a higher proportion of older people live in these homes than the population more generally, with over a million people over the age of 55 living in conditions that pose a risk to their health or safety.

It shouldn’t be too difficult to ensure all our homes meet a decent standard. A small number of homes require major and expensive remedial work, but the overwhelming majority need less than £3,000 to hit the mark. We know how to do it. We now need the political will to make it a priority. Apart from the benefits to the people living in the homes, investment of this kind is great for the economy, especially when so many of our skilled tradespeople are older. Imagine if they were part of training young people to learn these skills.


At a recent staff away day, we explored where we would ideally want to live in our later lives. This was not a stretch for me, although for some of our younger colleagues it is a long way into the future.

The point at which the conversation really took off for me was when we moved away from government definitions of decency and accessibility and began to explore the principles of what great homes for older people would be like. We agreed they needed light and space (by which we meant real space – our national obsession with number of bedrooms as opposed to space has led to us building the smallest new homes in Europe).

We agreed, too, that they needed to be as flexible as possible so that the space could be used differently as our needs change. We thought access to safe outdoor space was essential and that the homes should be digitally connected and in places that maximise the potential for social connection.

Of course, it took us just a few seconds to realise that this is true for virtually everyone. As a nation we have been dismal at moving away from three-bed boxes to thinking differently about what our homes should look like. In a world of technology and factory building, and as we build the new generation of homes we desperately need, we have a real chance to be bold.

Great, flexible homes with light and space, in the places where people want to live. Surely it’s not too much to ask?

David Orr is associate director – homes at the Centre for Ageing Better.