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.

 
 
 
 

To boost the high street, cities should invest in offices

Offices in Northampton. Image: Getty.

Access to cheap borrowing has encouraged local authorities to proactively invest in commercial property. These assets can be a valuable tool for cities looking to improve the built environment they offer businesses and residents.

Councils are estimated to have spent £3.8bn on property between 2013 and 2017, funded through the government’s Public Works Loan Board (PWLB) at very low interest rates. Offices accounted for half of this investment, and roughly a third (£1.2bn) has been spent on retail properties. And local authorities were the biggest investor group for UK shopping centres in the first quarter of 2018.

Why are cities investing? There are two major motivations.

First, at a time when cuts are squeezing council revenue budgets, property investments can provide a long-term revenue stream to keep quality public services up and running. Second, ownership of buildings in areas marked for redevelopment allows councils to assemble land more easily and gives them more influence over the changes taking place, allowing them to make sure the space evolves to meet their objectives.

But how exactly can cities turn property ownership into successful place-making? How should they adapt the buildings they invest in to improve the performance of the economies?

Cities need workers

When developing the city’s property offer, the aim should be to get jobs back into the city centre while reducing the dominance of retail space. For councils who have invested in existing retail space and shopping centres, in particular, the temptation may be to try and retain their existing use, with new retail strategies designed to reduce vacancies.

But as the Centre for Cities’ recent Building Blocks report illustrates, the evidence points to this being a dead-end. Instead, cities may need to convert the properties they own so they house a more diverse group of businesses.

Many city centres already have a lot of retail – and this has not offered significant economic benefit. Almost half (43 per cent) of city centre space in the weakest city economies is taken up by shops, while retail only accounts for 18 per cent of space in strong city centre economies. And many of these shops lie empty: in weaker city centres vacancy rates of high-street services (retail, food and leisure) are on average 16 per cent, compared with 9 per cent in stronger city economies. In Newport, nearly a quarter of these premises are empty, as the map below shows.

The big issue in these city centres is the lack of office jobs – which are an important contributor to footfall for retailers. This means that, in order to improve the fortunes of the high street, policy will need to tackle the barriers that deter those businesses from moving to their city centres.

One of these barriers is the quality of office space. In a number of struggling city centres, the quality of office space on offer is poor. But the low returns available for private investors mean that some form of public sector involvement will be required.


Ownership of buildings gives cities the opportunity to reshape the type of commercial space on offer. Some of this will involve improving the existing office stock available, some will involve converting retail to office, and some of will require demolishing part of the space without replacing it, in the short term at least. Without ownership of the land and buildings on it, this task becomes very difficult to do but will be a fundamental part of turning the fortunes of a city centre around.

Cheap borrowing has provided a way not only for local authorities to generate an income stream through property investment. but also opens up the opportunity to have greater control over the development of their city centres. For those choosing to invest, the focus must be on using ownership to make the city centre a more attractive place for all businesses to invest, rather than hoping to revive retail alone.

Rebecca McDonald is an analyst at the Centre for Cities, on whose blog this article first appeared.