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.

 
 
 
 

Ducks and the City: how birds thrive in urban spaces

A mandarin duck, possibly a distant relative of New York’s Hot Duck. Image: Stephen Jorgenson-Murray.

New York may be well known one of the most diverse, cosmopolitan places on Earth, but the arrival of one East Asian migrant in October 2018 still managed to surprise and delight the city. One lonely male mandarin duck – a gorgeous rust-red duck streaked with white and blue, native to Japan, Korea and East China – somehow found its way to Central Park and settled down on one of the ponds among the mallards and wood ducks to become the media sensation “Hot Duck”. Although not strictly wild in the birdspotting sense as it likely escaped from someone’s collection, the duck lives as free as, well, a bird among the skyscrapers of Manhattan.

A few months later, the mandarin’s native territory was graced by a rare visitor of its own when a European robin ended up in the heart of Beijing. Having shown up just when Britain was falling deeper into political crisis, Chinese birdspotters nicknamed it “Brexit refugee” and raced in from across the country to see what Brits would probably consider an incredibly ordinary bird.

A rash of unusual birds have hit the headlines after landing in cities lately – other recent examples include Melbourne’s “Goth Duck” (a tufted duck, a mainly northern European species never before seen in Australia) and the eagle owl that divebombed bald men in Exeter – but when they do, it’s always their rarity that makes them newsworthy, along with the incongruity of seeing a beautiful wild animal among concrete and litter. Normally cities aren’t home to anything more interesting than a dirty pigeon or a bloodthirsty seagull.

Right?

Moving in

Popular myth says London’s first ring-necked parakeets were released in Carnaby Street by Jimi Hendrix. It’s probably not true, but it’s one hell of a story. Image: Stephen Jorgenson-Murray.

Rome wasn’t built in a day, and nor was any other city. Thousands of years ago, wild birds discovered new opportunities on the edges of the first villages. Today the house sparrow is ubiquitous in just about every urban area in the world, but before the first house was built it lived in the dry grasslands of the Middle East, picking seeds out of the sandy soil. Then humans came along and started farming wheat; and whenever a grain fell from a mill or blew from a market stand, a sparrow was there to pick it up. As the technology of farming spread around the world, sparrows came along, too.

Other birds didn’t come by choice but were dragged in by humans. Thousands of rock doves, plump grey-striped birds that nest on cliffs, were caged up and brought into the new cities for their eggs, meat and uncanny ability to find their way home. Naturally, a few of these escaped, but quickly discovered that the walls of buildings were just as good for nesting as natural cliffs. The familiar pigeon was born.

More recently, many species of ducks and geese found a home in cities for the same reason, as have pets-gone-wild like the Indian ring-necked parakeets that brighten up London’s parks and the Javan mynas that chatter in Singapore’s streets.

Bohemian waxwings mainly live in the forests of Scandinavia, but in cold winters they will fly across the sea to British parks and gardens to feast on garden berries. No prizes for guessing where this one is. Image: Stephen Jorgenson-Murray.

As cities have changed, so too have the birds that lived there. Back when most meat was butchered in shops and markets, piles of skin and bone attracted huge flocks of scavengers like ravens and red kites. Now city streets are mostly free of scrap meat thanks to bin lorries, supermarkets and industrial meat processing; both species fled into the countryside, where they found themselves persecuted by farmers and gamekeepers, the red kite almost to the point of extinction. Now both birds are making a slow comeback.

On the other hand, parks and gardens have lured new species out of the woods and into the town with their sweet berry bushes and seed-filled bird feeders. Blue tits – tiny birds that in the forest prefer to pick spiders off oak trees – adapted especially well to garden life: in the days of milk rounds, the birds learned how to peck open bottle caps and sip at the cream inside. The birds’ behaviour has recently changed again because of the rise of supermarkets and the fall of dairy delivery, and it certainly won’t be the last time.

What do city birds think of us?

Herring gulls are as happy in a Latvian bus station as they are on a windswept beach. Happier, maybe. Image: Stephen Jorgenson-Murray.

If you walk in a forest you might well find yourself absolutely surrounded by bird song but unable to see where it’s coming from. Birds are shy and, unless they grew up on a desert island, they will fly away and hide at the slightest hint of a threat. They almost behave like programmed characters from a video game – they draw an imaginary circle around themselves (known as the “flight zone”) and if anyone enters that circle, they flee.

Urban birds consistently have a much smaller flight zone and will tend to let humans get much closer to them; and the longer a species has been urbanised, the more this radius shrinks. In the most extreme instance, urban birds will hop right up to someone who might feed them and even land on their hand. (In one of the best birding moments of my life, a parakeet in Hyde Park snatched a peanut from a tourist then landed right on my shoulder to eat it, staying there long enough to pose for a selfie).

If one bird invades another’s territory, things can get messy. Here, two magpies chase off a buzzard as its partner watches. Image: Stephen Jorgenson-Murray.

Then again, not all birds are that friendly. Many are very territorial, especially in the nesting season. Even medium-sized birds like vicious Australian magpies can cause eye injuries to people passing their nests; really big birds like swans can seriously injure people who get too close. Others, like the larger species of gulls, are just greedy and will attack people to steal their food.

Most birds aren’t quite that bold, but living close to humans has still affected their behaviour. Many species of birds are very intelligent – European magpies might be the cleverest non-mammal on the planet – and they’ve worked out how many of the systems of the city work. Pigeons can hop on-board trains for a lazier way to travel between feeding spots. Seagulls understand how to open automatic doors in order to raid branches of Greggs. Crows use passing cars to crack tough nuts, and will even wait at traffic lights to swoop in when the cars stop.

What do we make of city birds?

The robin was voted Britain’s favourite bird in a recent poll, which just goes to show what being small, cute and surprisingly aggressive can do for you. Image: Stephen Jorgenson-Murray.

Although we share our cities with a whole menagerie of wildlife, most of it is either shy and nocturnal, or prefers the dark, dirty places where humans rarely venture. Birds by contrast are inescapable – on any day on any city street you can expect to at least see a few pigeons flying overhead, or hear something singing from a nearby bush. For some people, this constant awareness has morphed into affection; for others, jealousy at sharing urban spaces with other species.


Even setting aside the risk of attack, birds can come into conflict with humans. Their droppings are not only unpleasant, but they can damage buildings and cause nasty lung diseases. Not every bird has a beautiful song either – a great tit squeaking away outside your bedroom window at 5am is bad enough, but spare a thought for the Australians who have kookaburras scream-laughing on their balconies. If waking you up wasn’t antisocial enough, big birds like herring gulls and Australian white ibises (better known as “bin chickens”) will rip open bin bags and fling the rubbish across your garden. The birds guilty of these indiscretions are generally classed as pests and many cities are fighting back – either by killing the birds or by taking eggs from their nests.

Herons eat fish from ponds and occasionally birds of prey will attack small pets. Urban pigeon keepers, angry after having a prize bird attacked by a sparrowhawk, occasionally try to poison or set cruel traps to kill hawks; but in general cities actually provide a safe haven for birds of prey. Scottish sparrowhawks seem to breed significantly better in cities, likely because there are so many other birds there to hunt.

In fact, many city councils are encouraging birds of prey as a natural way to control the population of pigeons and rats. Peregrine falcons – the fastest birds on the planet – are given protected nesting sites on church spires and skyscrapers and their every move is streamed on webcams. Harris hawks – native to American deserts – have been brought across the Atlantic to scare birds away from the tennis courts at Wimbledon.

Smaller, cuter birds don’t have any such image problems, and millions of Brits put bird seed in their gardens or feed the ducks at their local park. (I should add: if you do, please don’t give them bread, which lacks the vitamins birds need and causes a horrible disease called “angel wing”; seeds, vegetable peel or little bits of fruit are better.) Cities are increasingly recognised as places where you can spot interesting birds – right now, the bird tracking portal eBird lists no fewer than 289 species that have been seen in London – and the last couple of years have seen guides such as David Lindo’s How to be an Urban Birder and even scientific journals such as the Journal of Urban Ecology dedicated to the life of the town.

Save the birds

An American robin has a rest in Boston Common. American robins are in a completely different family to European robins, in case you ever wondered why the robin in Mary Poppins looked so messed up. Image: Stephen Jorgenson-Murray.

Although cities offer food and shelter, they also contain many threats. Glass windows are invisible death to birds flying at full speed – the exact number killed isn’t clear, but it might be as many as 30 million a year in the UK alone. Vehicles can also kill, especially in suburban areas where dense gardens meet busy streets.

Although city birds are protected from some of the predators that they would encounter in the countryside, there are still plenty of animals looking for a meaty meal – not least pet cats, which the RSPB estimates kill 55 million birds in the UK every year. 


These threats aren’t necessarily having an effect on bird populations as a whole – most birds lay more eggs than needed, and if one young bird is killed by a cat a sibling can take its place. The bigger risks come from changes to the environment itself. Pesticides, patios and over-neat lawns have reduced the number of insects crawling around, and therefore the amount of food available for birds like thrushes, starlings and sparrows.

In spite of how easy they are to observe, urban birds tend to be understudied compared to their rural cousins. The fact pigeons are so widespread means researchers often overlook them, but their ubiquity means that observing the birds can help scientists to track environmental changes and to compare cities that otherwise have little in common. Citizen science can help here – the bird tracking apps Birdtrack and eBird let anyone submit their bird sightings, and actually need more coverage of urban and suburban areas.

Thankfully, the idea of creating urban bird sanctuaries is now being taken seriously. Parks have a role to play, but many birds actually prefer the wild roughness of building sites and industrial land, where bare soil crawls with bugs and wildflowers grow gloriously high – ironically, brownfield sites can be as important to the ecosystem as pristine green belt. Perhaps the most spectacular example is the London Wetland Centre in Barnes. Just across the Thames from Hammersmith, this Victorian waterworks has been converted into marsh land and attracts huge flocks of water birds, many of which can’t be found anywhere else in London. In fact thanks to the reserve, a few birds such as the reed-dwelling bittern – which almost went extinct in the UK – are now easier to spot in London than in the countryside it.

Flying into the future

This blackbird probably doesn’t understand its rural cousins. Image: Stephen Jorgenson-Murray.

In his book Darwin in the City, the biologist Menno Schilthuizen suggests that we’ve been looking at blackbirds all wrong. European blackbirds were originally forest-dwellers eating berries and bugs from the ground. For this, they needed long, probing beaks and the ability to migrate in the winter when the soil froze hard. However, a few blackbirds – possibly initially those living in the hills around Rome – made their way into cities and found plentiful supplies of food year round.

Since they no longer needed to pry into the earth or the bark of trees, their beaks started to get shorter. Because food was available year round, their migration instinct was switched off. And because they needed to compete with traffic and the other noises of city life, their songs got louder. The city dwelling birds became incompatible with their forest dwelling ancestors; the changes to their beaks meant that their songs changed too, until they were effectively speaking different languages. There is a compelling case to be made that there isn’t just one species of blackbird, but two: the forest blackbird, Turdus merula, and the city blackbird, Turdus urbanicus.

Where the blackbird has led, other birds are sure to follow. British great tits are evolving bigger beaks that help them dig around in garden bird feeders and many urban birds have started singing the dawn chorus earlier to avoid traffic and aircraft noise and to take advantage of artificial streetlighting. City-dwelling pigeons even seem to be evolving darker feathers, probably because the dark pigment captures the toxic elements pigeons accidentally ingest when they peck at paint.

Nesting in coated metal gutters like this exposes pigeons to dangerous chemicals in the paint, and this pigeon’s dark feathers are likely an evolutionary response to that threat. Image: Stephen Jorgenson-Murray.

Birds are no longer just accidental wanderers into cities, nor are they just greedy opportunists: they are an integral part of urban ecosystems. Not only do cities need their birds – Increasingly, birds need their cities.

Stephen Jorgenson-Murray tweets at @stejormur. Many of the birds mentioned in this article tweet in a tree near you.