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.


How the rise of anti-crime politics caused lasting harm to Black Americans

"I see an awareness that has developed in the Black community in the last 10 years or so about how deeply racist the criminal justice system has become," James Forman Jr. says. (David McNew/Getty Images)

The police killing of George Floyd, and the protest movement that emerged from it, has reinvigorated a national conversation around reinventing criminal justice policy in the United States.

At the same time, reports that violent crime is rising in many US cities have resurrected talk of the much-disputed “Ferguson effect,” a theory put forward by law enforcement professionals, and some researchers, who argued that police slowdowns in the wake of the first wave of Black Lives Matter protests resulted in elevated rates of violent crime. President Donald Trump is trying to weaponise this narrative, paired with images of federal officers clashing with protesters in the streets of Portland, to wage a 1968-style backlash election campaign.

“People who want to mobilise a lock-them-up style of either policing or prosecution have tried to weaponise those short-term increases,” says James Forman Jr., professor of law at Yale Law School. “Criminologists will say you have to be very, very cautious about short-term movement [in crime statistics]. We don't know whether or not what we're seeing right now [with violent crime increasing] is going to sustain itself. But the fact is, it's here and people are talking about it.”

In 2018, Forman won the Pulitzer Prize in nonfiction for his book Locking Up Our Own: Crime and Punishment in Black America. Drawing on his experience as a public defender in Washington, DC, he traced the emergence of anti-crime politics in late 20th century Black communities. Forman showed how newly empowered Black politicians fought for policies they believed would protect and uplift Black Americans, but inadvertently contributed to mass incarceration. 

CityMetric recently caught up with Forman to discuss crime trends, where he sees reason for hope in this moment and how the Black political class’s attitude toward crime and punishment has shifted since the latter part of the 20th century. 

This interview has been edited and condensed. 

There is talk right now about a resurgence of crime and violence in American cities. We saw similar, more localised concerns after the initial 2015 Black Lives Matter protests in Ferguson and Baltimore. Do you fear this could reinvigorate the kind of politics you describe in your book among segments of the Black community and political class?

I fear that it could be reinvigorated nationally and also in the Black political class. Look at the political conversations that are happening in Atlanta right now, for example, a city that also has seen a short-term uptick in crime as it is a site of a lot of protests about George Floyd and Breonna Taylor on the national level, as well as Rayshard Brooks and Ahmaud Arbery more locally in Georgia.

I think that you can already see in some of the language of the local elected officials this idea that we have to be very careful about pulling back. [They are saying] “while the protesters may make some valid points, we can't risk returning to the ‘80s and ‘90s.” Those decades really traumatised the United States, and particularly traumatised Black communities. There's a deep fear about returning to the levels of the violence that we saw in the crack years.

You write a lot about class divides among Black Americans, where middle income and elite Black people don't suffer as much from extremely punitive policies. They also have closer ties to the politicians who are creating these policies. There are very specific groups of people, even in marginalised communities, whose voices are heard.  As a result of these dynamics, you write about Black politicians fighting for things like mandatory minimum prison sentences or against decriminalising marijuana. Is there still that disconnect between those who suffer the most from criminal justice policies and those who are actually heard in political discourse?  

Let me just say a caveat, that when we talk about class divisions in the Black community it's important to hold two truths in our head at the same time. Bruce Western and others have shown the way in which class, educational status, income can dramatically reduce the likelihood of being hardest hit by the criminal system – namely incarcerated. Middle class and upper middle class Black people get some measure of protection. It's also true at the same time that Black people of all classes are worse off relative to their class counterparts in the white community. 

One area where class is least protective is policing and police stops. The police do not know how many degrees you have. They don't know how much money you have in your bank account. I want to be very clear that in making this point about class, I'm not making the argument that race or racism don't matter in this context. 

In terms of how it plays out now, I see an awareness that has developed in the Black community in the last 10 years or so about how deeply racist the criminal justice system has become. Twenty or 30 years ago they had a consciousness, but there's levels of understanding. Many of the people I write about in the book wanted to promote the interests of the Black community. They weren't motivated by indifference or callousness. When presented with mounting evidence of how awful this system has been in Black lives, they're reconsidering and recalibrating. 

Lots of former elected officials have said to me some version of “I didn't know at the time and I appreciate that you showed us in our full complexity. I appreciate that you showed the pressures we were under. If I had known then what I know now, maybe I would have been less quick to go along with some of these harsh measures.” 

The second thing that has affected the Black political class has been the emerging movements, led by Black people in particular and led by young people. They not only educated leaders, but pressured them and made them understand that there is a political cost. If you're not moved by the moral argument, then you'll be moved by the political argument. You'll be moved by the people protesting outside the office of District Attorney Jackie Lacey in Los Angeles, for example, where Black Lives Matter LA has held, I believe, a year of consecutive protests against a Black district attorney who has had really some of the worst practices.

From what I can tell, she's been pressured by the movement to change some of her positions on important issues like prosecution of low-level drug offenders, for example, and the aggressiveness with which she prosecutes police officers for acts of violence.

What do you make of the calls to defend or even abolish the police?

What I find so compelling about abolition, initially in the prison context and extended to the police as well, is that it shifts the conversation and forces us to go through experiments in which we imagine what it would take to build that world. I think that exercise is very important, because it pushes us further than we are naturally inclined to go. Cultivating a broader imagination is an incredibly important part of this work, because as you know from my book, often it was lack of imagination that caused people to fall back on [punitive policies]. 

That's what caused D.C. Councilmember David Clarke to call the police rather than public health experts when he was overwhelmed with letters about heroin addicts in public space. He was anti-drug war, but he couldn't imagine responding to a call for help with heroin addicts with anything other than police. That's a very common move from even really good and progressive people. 

People who are for defunding, for abolition, are absolutely right about reinvesting that money into alternative structures that support communities. But the reinvestment part doesn't follow naturally from the terms. We might want to come up with a term that captures the new stuff we want to do. I think that's particularly important because one of the reasons Black communities have ended up supporting more police is that Black communities have always wanted their fair share of the resources.

Then, the evidence suggests the United States has too many police officers doing prophylactic, preventative, or stop-and-frisk style policing. The style of policing that leads to district level harassment, pulling people over for no reason. But we have too little investment in the parts of police departments that investigate unsolved crimes. I'm talking about the investigator or the detective who comes to your house after there's been a robbery, an assault, a rape, or homicide. 

As compared to European countries, in the United States we actually underinvest in those parts of our police departments. Jill Leovy’s book Ghettoside shows this in dramatic detail. She describes an LAPD that's stopping and frisking Black drivers wantonly and yet the homicide detectives are still relying on a fax machine and the fax machine is broken. They have to go with their own money to Staples to buy a printer. Meanwhile, other aspects of the department are kitted out in this ridiculous riot gear that makes them look like they're in Fallujah. 

That under investment is particularly damaging to Black communities because we're disproportionately victimised by crime. Because of racism and this allocation of resources, the police are less likely to respond in Black communities. The kids I used to work with in the charter schools in DC, we talk about no snitching, but one of the reasons they would never call the police after they'd been victimised by crime is they would say, “They're not even going to come. You're wasting time.” 

I did a Q&A with Jill Leovy too and her argument is one I've struggled to articulate in our present moment. She argues the state doesn't have a monopoly on violence in low-income Black neighbourhoods, because investigations of violence are deemphasised and crime victims or their loved ones often take retribution into their own hands.  But right now, establishing or preserving the state's monopoly on violence isn't an appealing talking point. 

Yes, this is another thing nobody's talking about. Whatever we're going to do instead of the police has to be accountable to the public. The best, most direct way to have accountability is to have the individuals be public employees. As long as we have 300 million guns in this country at least some of those state employees are going to themselves be armed. It's unreasonable to ask them to do the job without it. Not as many need to be armed as are armed now, but some of them need to be. But they can't be hiding behind union contracts or civil service protections which make it impossible to remove even the worst performing, most abusive officers. 

We can not call them police if we want to. That's semantic, but maybe symbolism matters. But those people have to be state employees. They can work with community-based nonprofits, but there are also communities that don't have as robust of a nonprofit network, and they deserve protection too. These [community] groups have to be accountable to the state and, when they don't exist, the state has to be there. 

Progressives get all the points I just made when it's applied to education. The notion that things be public and accountable to the state is understood when it comes to schools. It's exactly why so many people on the left are opposed to charter schools, because they say they don't have public accountability. They want these things to be a state function. But this point about the difficulty in removing this entirely from the hands of the state is, I think, one that liberals and progressives understand from other contexts.

Jake Blumgart is a staff writer at CityMetric.