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 build its emerging “megaregions”, the USA should turn to trains

Under construction: high speed rail in California. Image: Getty.

An extract from “Designing the Megaregion: Meeting Urban Challenges at a New Scale”, out now from Island Press.

A regional transportation system does not become balanced until all its parts are operating effectively. Highways, arterial streets, and local streets are essential, and every megaregion has them, although there is often a big backlog of needed repairs, especially for bridges. Airports for long-distance travel are also recognized as essential, and there are major airports in all the evolving megaregions. Both highways and airports are overloaded at peak periods in the megaregions because of gaps in the rest of the transportation system. Predictions for 2040, when the megaregions will be far more developed than they are today, show that there will be much worse traffic congestion and more airport delays.

What is needed to create a better balance? Passenger rail service that is fast enough to be competitive with driving and with some short airplane trips, commuter rail to major employment centers to take some travelers off highways, and improved local transit systems, especially those that make use of exclusive transit rights-of-way, again to reduce the number of cars on highways and arterial roads. Bicycle paths, sidewalks, and pedestrian paths are also important for reducing car trips in neighborhoods and business centers.

Implementing “fast enough” passenger rail

Long-distance Amtrak trains and commuter rail on conventional, unelectrified tracks are powered by diesel locomotives that can attain a maximum permitted speed of 79 miles per hour, which works out to average operating speeds of 30 to 50 miles per hour. At these speeds, trains are not competitive with driving or even short airline flights.

Trains that can attain 110 miles per hour and can operate at average speeds of 70 miles per hour are fast enough to help balance transportation in megaregions. A trip that takes two to three hours by rail can be competitive with a one-hour flight because of the need to allow an hour and a half or more to get to the boarding area through security, plus the time needed to pick up checked baggage. A two-to-three-hour train trip can be competitive with driving when the distance between destinations is more than two hundred miles – particularly for business travelers who want to sit and work on the train. Of course, the trains also have to be frequent enough, and the traveler’s destination needs to be easily reachable from a train station.

An important factor in reaching higher railway speeds is the recent federal law requiring all trains to have a positive train control safety system, where automated devices manage train separation to avoid collisions, as well as to prevent excessive speeds and deal with track repairs and other temporary situations. What are called high-speed trains in the United States, averaging 70 miles per hour, need gate controls at grade crossings, upgraded tracks, and trains with tilt technology – as on the Acela trains – to permit faster speeds around curves. The Virgin Trains in Florida have diesel-electric locomotives with an electrical generator on board that drives the train but is powered by a diesel engine. 

The faster the train needs to operate, the larger, and heavier, these diesel-electric locomotives have to be, setting an effective speed limit on this technology. The faster speeds possible on the portion of Amtrak’s Acela service north of New Haven, Connecticut, came after the entire line was electrified, as engines that get their power from lines along the track can be smaller and much lighter, and thus go faster. Catenary or third-rail electric trains, like Amtrak’s Acela, can attain speeds of 150 miles per hour, but only a few portions of the tracks now permit this, and average operating speeds are much lower.

Possible alternatives to fast enough trains

True electric high-speed rail can attain maximum operating speeds of 150 to 220 miles per hour, with average operating speeds from 120 to 200 miles per hour. These trains need their own grade-separated track structure, which means new alignments, which are expensive to build. In some places the property-acquisition problem may make a new alignment impossible, unless tunnels are used. True high speeds may be attained by the proposed Texas Central train from Dallas to Houston, and on some portions of the California High-Speed Rail line, should it ever be completed. All of the California line is to be electrified, but some sections will be conventional tracks so that average operating speeds will be lower.


Maglev technology is sometimes mentioned as the ultimate solution to attaining high-speed rail travel. A maglev train travels just above a guideway using magnetic levitation and is propelled by electromagnetic energy. There is an operating maglev train connecting the center of Shanghai to its Pudong International Airport. It can reach a top speed of 267 miles per hour, although its average speed is much lower, as the distance is short and most of the trip is spent getting up to speed or decelerating. The Chinese government has not, so far, used this technology in any other application while building a national system of long-distance, high-speed electric trains. However, there has been a recent announcement of a proposed Chinese maglev train that can attain speeds of 375 miles per hour.

The Hyperloop is a proposed technology that would, in theory, permit passenger trains to travel through large tubes from which all air has been evacuated, and would be even faster than today’s highest-speed trains. Elon Musk has formed a company to develop this virtually frictionless mode of travel, which would have speeds to make it competitive with medium- and even long-distance airplane travel. However, the Hyperloop technology is not yet ready to be applied to real travel situations, and the infrastructure to support it, whether an elevated system or a tunnel, will have all the problems of building conventional high-speed rail on separate guideways, and will also be even more expensive, as a tube has to be constructed as well as the train.

Megaregions need fast enough trains now

Even if new technology someday creates long-distance passenger trains with travel times competitive with airplanes, passenger traffic will still benefit from upgrading rail service to fast-enough trains for many of the trips within a megaregion, now and in the future. States already have the responsibility of financing passenger trains in megaregion rail corridors. Section 209 of the federal Passenger Rail Investment and Improvement Act of 2008 requires states to pay 85 percent of operating costs for all Amtrak routes of less than 750 miles (the legislation exempts the Northeast Corridor) as well as capital maintenance costs of the Amtrak equipment they use, plus support costs for such programs as safety and marketing. 

California’s Caltrans and Capitol Corridor Joint Powers Authority, Connecticut, Indiana, Illinois, Maine’s Northern New England Passenger Rail Authority, Massachusetts, Michigan, Missouri, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas, Vermont, Virginia, Washington, and Wisconsin all have agreements with Amtrak to operate their state corridor services. Amtrak has agreements with the freight railroads that own the tracks, and by law, its operations have priority over freight trains.

At present it appears that upgrading these corridor services to fast-enough trains will also be primarily the responsibility of the states, although they may be able to receive federal grants and loans. The track improvements being financed by the State of Michigan are an example of the way a state can take control over rail service. These tracks will eventually be part of 110-mile-per-hour service between Chicago and Detroit, with commitments from not just Michigan but also Illinois and Indiana. Fast-enough service between Chicago and Detroit could become a major organizer in an evolving megaregion, with stops at key cities along the way, including Kalamazoo, Battle Creek, and Ann Arbor. 

Cooperation among states for faster train service requires formal agreements, in this case, the Midwest Interstate Passenger Rail Compact. The participants are Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, and Wisconsin. There is also an advocacy organization to support the objectives of the compact, the Midwest Interstate Passenger Rail Commission.

States could, in future, reach operating agreements with a private company such as Virgin Trains USA, but the private company would have to negotiate its own agreement with the freight railroads, and also negotiate its own dispatching priorities. Virgin Trains says in its prospectus that it can finance track improvements itself. If the Virgin Trains service in Florida proves to be profitable, it could lead to other private investments in fast-enough trains.

Jonathan Barnett is an emeritus Professor of Practice in City and Regional Planning, and former director of the Urban Design Program, at the University of Pennsylvania. 

This is an extract from “Designing the Megaregion: Meeting Urban Challenges at a New Scale”, published now by Island Press. You can find out more here.