1. Transport
April 23, 2015

Why do so many public transport networks use grid systems?

By Jarrett Walker

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. 

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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.

This article is from the CityMetric archive: some formatting and images may not be present.
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