Why do so many public transport networks use grid systems?

An illustrated map of Milwaukee, 1872. Image: Howard Heston Bailey, public domain.

Why do transit planners love grids? Now and then you'll even hear one muttering about “grid integrity” or “completing the grid”. What are they talking about?

Suppose you're designing an ideal public transit system for a fairly dense city where there are many activity centres, not just one big downtown area. In fact, you don't want to give preferential treatment to any point in the city: instead, you want people to be able to travel from literally anywhere to anywhere else by a reasonably direct path, at a high frequency.

Everybody would really like a frequent service from their home to everywhere they ever go, which is pretty much what a private car is. But money isn't infinite, so the system has to deliver its outcome efficiently, with the minimum possible cost per rider. What would such a system look like?

Well, you already know that to serve a two-dimensional city with one-dimensional transit lines, your system has to be built on connections, and for that you need high frequencies. Frequency is expensive, so it follows that you need to minimize the total route distance so that you can maximize the frequency on each. That means you can't afford to have routes overlapping each other.

Feel free to play with this problem yourself, but what you’ll find is that the answer is a grid: parallel lines and another set of the same lines perpendicular to them.

In an ideal grid system, everyone is within walking distance of one north-south line and one east-west line. So you can get from anywhere to anywhere, with one connection, while following a reasonably direct L-shaped path. 

If your city street network is a grid, the path is often exactly the same way you'd make the trip if you were driving. For this trip to be attractive, all the services have to be very frequent, so that you don't have to wait long for the connection.

The spacing between parallel lines in our ideal grid is exactly twice our maximum walking distance. So if we're thinking in terms of ordinary local stop bus lines, maximum walking distance is about 1/4 mi or 400m, so our ideal spacing between parallel lines is 1/2 mi or 800m. But in fact, successful grid systems run really frequently, so we can afford walking distances a little larger than that, up to say 1 km or about 3/4 mile. 

(I'm assuming for the moment that these are local-stop services, so that when you've walked to a line there's a stop nearby. You could also imagine a grid of rapid or limited-stop bus services, such as Los Angeles has, or even a grid of underground or elevated subways, as in Paris or Berlin. People will walk still further for those, but this doesn't let you push the parallel lines further apart, because the need to walk to widely spaced stations, rather than closely spaced stops, consumes some of that extra walking distance.)

The intrinsic efficiency of grids gives an advantage to cities that have arterial streets or potential transit corridors laid out in a grid pattern, especially if they have many major destinations scattered all over the city. If your city or a part of it looks like that, you have a huge structural aid in evolving into a transit metropolis. Los Angeles and Vancouver are two of the most perfect transit cities I've seen, in their underlying geography, because they have these features. More on this aspect of both cities shortly. 

Note that the grid works because people can walk to both a north-south and an east-west line, and for this reason, cities or districts with labyrinthine local street patterns that obstruct pedestrians (Las Vegas, most of Phoenix, and much of suburban Southern California, for example) will have a harder time becoming transit-friendly, even though they do have a grid pattern of major arterial streets, because pedestrians can't get out to the grid arterials easily, or cross them safely.

Grids are so powerful that dense cities that lack a grid network of streets often still try to create a grid network of transit. Gaze at a schematic map of the Paris Métro for a bit and its underying grid pattern will start to emerge: most lines flow pretty consistently either north-south or east-west across the city, and while they can't remain entirely parallel or evenly spaced as they snake through this city of obstacles, you can see that on some level, they're trying to.

Or look at San Francisco. The basic shape of the city is a square about seven miles on a side, with downtown in the northeast corner. Because the downtown area is a huge transit destination, there are many routes from all parts of the city converging on it, in a classic radial pattern. But under the surface, there's also a grid. San Francisco's published network map is too complicated to reveal it easily, but you can see the grid if you look at a few schematics of individual routes. For example, Lines 23-Monterey and 48-Quintara 24th St are east-west elements.

Notice how these two lines remain largely parallel as they cross the city. This is interesting because San Francisco's street network has a lot of small grids, but no prevailing citywide grid. In fact, a major ridgeline runs north-south through the geographic centre of the city, and the arterial network is very un-gridlike as it follows the steep terrain. As a result, these lines have to twist a bit to get over it using the available streets: the 48 has to twist again to get over Potrero Hill on the east edge of the city, where there is no available east-west street. Yet they keep trying. 

Notice too that both routes try to get all the way across the grid before they end, so that almost all end-of-line points are on edges of the city. This is a common feature of good grid design, because it maximizes the range of places you can get to in just one connection. If you look at the abstract grid diagrams earlier in the post, you can see how they'd work less well if some lines in the grid ended without intersecting every one of the perpendicular lines. You'd have fewer options for how to complete a trip with a single connection.

So if they’re so great, why aren't all frequent networks grids? The competing impulse is the radial network impulse, which says: “We have one downtown, everyone is going there, so just run everything to there." Most networks start out radial, but some later transition to more of a grid form, often with compromises in which a grid pattern of routes is distorted around downtown so that many parallel routes converge there. 

You can see this pattern in many cities, including Portland. Many of the lines extending north and east out of the city center form elements of a grid, but converge on the downtown. Many other major routes (numbered in the 70s in Portland's system) do not go downtown, but instead complete the grid pattern. This balance between grid and radial patterns was carefully constructed in 1982, replacing an old network in which almost all routes went downtown.

Another way of distorting the grid to favour downtown is suggested by Portland's two prominent diagonal boulevards, Sandy in the northeast and Foster in the southeast. These lines, suggested if not mandated by the available arterials, follow a more direct path into downtown at the expense of being slightly less useful for other kinds of trips within the grid.

These diagonals and distortions are essentially elements of a competing type of grid: the classic  “radial “ or polar grid, also called a  “spider web “

The spider web assumes a single point of primacy, downtown, and organizes a grid around that primacy. If you zoom in on some part of the spider web, you may find that it works well enough as a standard grid. For example, you may be able to make a reasonably direct trip between non-downtown points by using one of the circle lines in combination with one of the radial lines.

But it won't be as direct as it would be in a standard grid. More important, the spider web is only efficient if downtown is so predominant that it can justify the huge amount of service converging there. The spider web also has problems further out, because as the radial lines get further and further apart the grid effect gets weaker and weaker. 

You can tell a lot about a city by looking at the tension between standard grid elements and radial or “spider web” elements.

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. All images courtesy of the author.


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.