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Milan, 1913

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Once a road, always a road. That’s the gist of a recent paper that studied 14 different municipalities in the Groane region of Italy near Milan. In cataloging 174 years of road construction, the study’s authors discovered that nearly 90 percent of the regions 100 most vital routes today were already present in 1833.

The researchers also uncovered evidence that the layout and characteristics of road networks are indicative of the age in which they were built. This is nothing new. Take a look at any metro region surrounded by a postwar subdivision—stick straight roads of the late 19th and early 20th centuries give way to ever more writhing tangles of spaghetti. What’s new is that this study claims that top-down planning didn’t drive the changes. Rather, the researchers say Groane’s roads reflect broader societal changes, that the unique circumstances of each era—agricultural through modern—shape road networks more than central planning—or lack thereof.

To arrive at that conclusion, physicist Marc Barthélemy and his colleagues digitized roads using maps and aerial photographs from seven different dates between 1833 to 2007. They threw the resultant vectors into a geographic information system, or GIS, and then distilled primal graphs—simplified maps that show only roads (called “links” in graph theory) and intersections (“nodes”).

Between 1833 and 2007, the number of intersections grew proportionately with population. The number of intersections skyrocketed—there were only 255 in 1833 but over 5,000 in 2007—but the number of connections remained relatively constant at 2.7 on average. The number of roads also increased linearly with the number of intersections. It’s almost as if the expansion of Groane’s social network was mirrored in its transportation corridors.

How those roads interacted with each other changed through time. In the early days, many roads either intersected another mid-link—forming a T-junction—while the others simply petered out in a dead end. Main drags radiated out from town centers like spokes on a wheel. Congruent 4-cornered intersections were rare. Yet as time progressed and cities spread into the countryside, the previous radial expansion gave way to the grid. In other studies, the advent of the grid was attributed to the arrival of master planning, but here in Groane, Barthélemy and his colleagues note that urban planning was never the region’s strong suit. Groane, they write, “never witnessed any large scale planning whatsoever.”

It is because of Groane’s lack of central planning that Barthélemy and his colleagues are able to draw their conclusion, that road networks morphed not because of changes in our approach to planning but because of changes in society as a whole. In essence, they assert that changes to the network were not consciously done.

It’s not surprising, really. Roads are built to handle the traffic of their time. When navigating Cambridge’s labyrinthine streets, I’m constantly reminded that they were built for horse and carriage, not a horseless carriage. The demands of the automobile are sufficiently different from horse or foot traffic. Their greater speeds require straighter rights-of-way. Intersections need to be clear and predictable. Navigation also needs to be simplified—drivers moving at 10 miles per hour have more time to look for their next turn than those moving three times faster. The grid tackles these problems with aplomb.

Road networks are a product of the processes that created them, whether that be wagon traffic from farm fields plodding to town or taxi cabs streaming out from downtown. Discerning process from pattern is also the domain of another field—landscape ecology. Landscape ecologists sweat the details of spatial configuration to learn what ecological processes are at work. The laws of landscape ecology apply just as well in the city as they do in the natural world. The city is nothing but an anthropogenic ecosystem.

Ever since Geoffrey West and his colleagues uncovered the mathematics behind why big cities are economically successfully—but also crime ridden—it has been popular to search for formulae that describe urban processes and city development. This paper by Barthélemy and his colleagues is but the latest addition to a growing literature. By themselves, these discoveries are clever and insightful. But the interesting stuff will happen when urban planning completes the transition from an observation-based science to a mathematical one, much as ecology did in the recent past. Then we’ll have a real sense of how these models will change our understanding of cities.

Map scanned by University of Texas PCL Map Collection.

Source:

Strano, E., Nicosia, V., Latora, V., Porta, S., & Barthélemy, M. (2012). Elementary processes governing the evolution of road networks Scientific Reports, 2 DOI: 10.1038/srep00296

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Map of photos taken in Minneapolis and St. Paul, Minnesota

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Cities were, for thousands of years, distinct and easily identifiable entities. You were either in the city or in the country. Medieval cities took this to the extreme, building walls to make explicit the distinction. Johann Heinrich von Thünen systematized the idea in 1826 when he sketched a hypothetical map that, when simplified, looked like a bow-and-arrow target. The city sat in the center and was surrounded by rings of successively less valuable farmland. It was all very orderly and very German. And for a while it did a good job describing the relationship between the city and the hinterland.

Then came the railroads and automobiles that shot holes through von Thünen’s well-organized bullseye. And in places where two cities were less than a few dozen miles apart, even the boundary between the two became blurred. Today, it’s not uncommon to find metropolitan areas with two, three, even four major cities anchoring them.

Von Thunen's model of land use

Multi-city metros would seem to be a many-headed monster, riddled with contrary opinions and paralyzed by indecision. But that doesn’t alway seem to be the case. As far as labor productivity is concerned, multi-city metros—or polycentric metros, as the literature calls them—may have a distinct advantage. A study of all metropolitan areas in the United States with populations above 250,000 by Evert Meijers and Martijn Burger shows that productivity is higher in metros with more than one city. The effect is especially pronounced among smaller metro areas.

Meijers and Burger speculate that’s because smaller cities tend to have smaller problems—less traffic, lower crime rates, and so on. By splitting the problems up among a few cities, polycentric metros can host a large population without experiencing the problems of a similarly sized, monocentric metro.

But the advantages of multi-city metros diminish as the entire area’s population grows. It’s as though the larger entity needs one place to focus its efforts. So a metro area with two cities, each one-half the size of London, wouldn’t necessarily be more productive than London itself.

Multi-city metros also fall short on other critical parts of city life—cultural and leisure opportunities. Cultural outposts like opera houses and art museums benefit greatly from larger populations, which typically contain more benefactors, both wealthy and otherwise. The same goes for sports teams. Every city would like one for themselves. Say Ft. Worth wants to build an art museum. It’s probably not going to attract some donors from Dallas, who would rather see one built in their city. Chicago doesn’t have such a problem. Monocentric metros don’t have to worry about sharing.

As cities’ borders swell, multi-city urban agglomerations are probably going to be more and more common. Even within existing metropolitan areas, smaller cities could rise to prominence. Minneapolis and St. Paul, for example, have had to contend with the rise of Bloomington. The key will be for leaders to learn to work together, coordinating efforts rather than stepping on each other’s toes.

Sources:

Meijers, E. (2008). Summing Small Cities Does Not Make a Large City: Polycentric Urban Regions and the Provision of Cultural, Leisure and Sports Amenities Urban Studies, 45 (11), 2323-2342 DOI: 10.1177/0042098008095870

Meijers, E., & Burger, M. (2010). Spatial structure and productivity in US metropolitan areas Environment and Planning A, 42 (6), 1383-1402 DOI: 10.1068/a42151

Map of the Twin Cities by the inimitable Eric Fischer.

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Chicago streets at night

Pushing high density living may seem like a good way to get people out of their cars—saving them money, curbing emissions, and reducing oil dependence—but densification may not be a silver bullet, according to one recent study. The authors dug into the National Household Transportation Survey to examine per household vehicle ownership rates, vehicle miles traveled (VMT), and fuel consumption. While the results are by no means comprehensive or conclusive, they suggest that only the steepest increases in density could reduce car usage.

This post was chosen as an Editor's Selection for ResearchBlogging.orgDespite a correlation between density and car usage, other factors seem to play more important roles. Density is responsible for a fraction of annual VMT; increasing density by 1,000 housing units per square mile—a titanic leap, given that the average household is 2.6 people—reduces VMT by just 1,171 miles, all else being equal. Since that the average one-driver household in the study tacks on 10,100 miles per year, that represents just over an 11 percent drop in annual mileage.

housing density and Vehicle Miles Traveled

If you look at the numbers another way, the case for density reducing car usage looks even more tenuous. VMT only really declines substantially at the highest housing density—over 5,000 units per square mile, or about the same as Chicago. To halve VMT of the highest mileage households, you would need to increase housing density in those areas by 20- to 100- fold.

The inflexibility of our automobile usage boils down to a few factors, with work being the most important. The more workers in a household, the more drivers, and the more drivers, the more miles. A one-driver household, as noted above, tallies 10,100 miles per year; a two-driver household racks up 18,800 miles; three drivers, 33,900; four drivers, 47,700.¹ We are, by and large, beholden to our cars because we are beholden to our jobs. After that, driving increases as a result of income (richer people drive more), number of children (more and larger cars), education (higher education means more cars), and people’s life stage (households with older children have more cars).

While higher housing density doesn’t seem to reduce VMT, it does drive down fuel consumption. Households in the 50 to 250 houses per square mile range use 1,650 gallons of fuel annually, the most of any group. Every other group uses far less fuel. In the big cities, fuel usage drops to 690 gallons per household per year.² The reason? People with the space to use pickup trucks, SUVs, and vans tend to buy them more than people who live and drive on tighter city streets—they typically drive smaller, more fuel efficient vehicles. Yet this trend could be changing as we speak. Small car purchases have been increasing across the country, and anecdotally at least, I can confirm that large pickup trucks are harder than ever to sell these days.

fuel consumption and Vehicle Miles Traveled

One of the main arguments behind higher density living is that it will reduce our carbon footprint. While density may be a better long term solution, right now the most expeditious approach is to increase fuel economy. Rebuilding neighborhoods will take decades. In that time, most people will buy at least a handful of new cars, primarily for commuting to work. It would be great if everyone had access to mass transit, but for many, mass transit isn’t just a poor option, it isn’t an option at all. Those who do travel by bus or train today may only be a job change away from having to drive. Modern life demands mobility, and few things are better at providing that than the automobile.

¹ The increase from one to two drivers probably reflects some combining of trips by couples or roommates. The sharp increase from two to three drivers is probably the result of a family’s children driving to school or work.

² The lone outlier is areas below 50 houses per square mile, where households use 1,200 gallons per year. They probably have fewer nearby destinations, and so stay home more often.

Source:

Brownstone, D., & Golob, T. (2009). The impact of residential density on vehicle usage and energy consumption Journal of Urban Economics, 65 (1), 91-98 DOI: 10.1016/j.jue.2008.09.002

Photo by dsearls.

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Accessory dwelling unit

Sprawl, in many cases, is in the past tense. It’s already happened. Though it continues in plenty of places, it’s most problematic where it already exists. For booming regions, sprawlish suburban rings threaten to choke the central city with snarled traffic and lean tax rolls. Rather than throw their hands up in disgust and move on to a city that “gets” it, a group of planners and architects have focused on retrofitting diffuse neighborhoods and intensifying four lane boulevards. In the process, there are myriad problems to tackle, but I’d like to tackle one right now: What to do with suburban housing.

Density is suburbia’s saving grace and Achille’s heal. Who wouldn’t want to be ensconced in their own private garden? But that same dispersed living isolates people and causes many of the headaches of daily life. Sprawl retrofit really means higher density living, which also means more shops close at hand and less time behind the wheel. It’s no secret that many suburban lots would easily support another house. Slapping another behind the existing one is a simple way to double the density. It’s a common approach to sprawl retrofit, and one raised at the 19th Congress for the New Urbanism. But if erecting backyard residences solves one set of headaches, it risks inflicting more in the process.

Suburban homes were built with very specific ideals in mind. The front of the house is the public side, presenting its widest part to the street, shielding the backyard and creating a private area where people can relax in relative peace. While many early car-centric suburban houses reacted tepidly to the backyard—the best they offered were a few more windows and a back door—we soon became practiced in the art of suburbia. Large picture windows, sliding doors, four-season rooms, and covered porches quickly festooned the private side. The backyard became more than an outdoor amenity—it was a private garden to be enjoyed from the indoors, too.

Hacking a lot in two and adding a second residence could destroy one of the few things many suburban homes have going for them—a sense of privacy. The attractiveness of this feature should not be underestimated. Research suggests that open views are help ensure a feeling of privacy, even more so than actual proximity. The bad of suburbia—houses turning their backs on the street—was driven by a noble aspiration—homes embracing the yard. No one wants their view to embrace another home, no matter how beautifully designed.

And so a lynchpin of sprawl retrofit—boosting the number of houses—may be the most difficult to achieve. That’s not to say it cannot be done, but it will require careful consideration to do properly. Suburbia may be, in many ways, a consequence of bad planning, but it got one thing right—the yard. We crave green, open space, and suburban lots with suburban houses have it in spades. It may have given us too much, but we shouldn’t take it away in too much haste lest we compound the mistakes of the past.

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Photo by faceless b.

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I-80 and M2

Travel can be revealing. In many cases, “where” can answer as much about a person as “who.” Much of who we are is tied up in what sorts of stores we frequent, where we work, and where we go for fun. While that sounds creepy—especially given the recent furor over smartphones storing location information—city-wide travel trends can help explain a lot about where we live.

A study comparing British and American travel habits discovered that trip distances in the United States predictably lengthen as the population thins out. In Great Britain, though, that relationship isn’t as strong, in part due to its greater population density. But when the researchers filtered out differences in income and density between Great Britain and the U.S., the British still traveled less.

The reason behind the travel discrepancy, the researchers suspect, lies in the strict separation of space for living, working, and shopping that is common in the U.S. To arrive at that conclusion, they had to use a bit of mathematical cleverness to make an apples-to-apples comparison. The model compared British and American cities of similar population and density, but shuffled the contents of the American ones until their general urban form resembled their ale-swilling counterparts. This meant that shops were now next to houses, grocery stores located just down the street, and pubs scattered about town. The results suggest that the jumbled urban land uses in England, Scotland, and Wales keep the lid on travel distances.

The British also get out from behind the wheel more often, using their feet for something other than operating gas and brake pedals. Whereas under 7 percent of trips in the U.S. are taken on foot, nearly one-third are in Great Britain. And while almost 90 percent of trips in the U.S. are made by car; only 58 percent are in Britain. More people in Britain take buses and trains, too. This modal flexibility could help explain why British of different incomes tend to take similar numbers of trips, unlike in the U.S. where wealthier people step out more often. In both countries, though, wealth is still tightly correlated with mobility, which is partially reflective of people’s ability to own, maintain, and fuel automobiles for such trips.

The one odd bit the researchers discovered was that people in smaller metropolitan areas—with populations between 250,000 to 500,000—traveled more frequently than larger ones. In theory, residents of large metro areas should have more available destinations and ways to get there, which left them stumped as to why big city people made fewer trips. My suspicion? Smaller metro areas may be a sweet spot in the spectrum, with just enough destinations and modes of transportation without the congestion present in big cities.

Source:

Giuliano, G., & Narayan, D. (2003). Another look at travel patterns and urban form: The US and Great Britain Urban Studies, 40 (11), 2295-2312 DOI: 10.1080/0042098032000123303

Photos by Minesweeper and Loganberry.

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Night traffic in New York City

It’s generally accepted as fact that people in big cities drive less. Things are closer together there, making it easier to walk to the store for a gallon of milk. For longer trips, mass transit is also an option. But boiling all that common sense down to a single number is difficult. And though our data-happy society can overdo it at times, numbers can reveal some surprising facts that simple observations otherwise wouldn’t—like the fact that increasing density reduces the amount people have to drive, but not by as much as you might suspect.

Determining precisely how far people drive—vehicle mile travelled (VMT) in the parlance of the field—is important for a number of reasons. With it, we can gauge greenhouse gas emissions from automobiles, allocate resources for road repairs or expansions, and refocus efforts to reduce car dependency. VMT is especially important in California, where the Global Warming Solution Act (also known as AB 32) calls on the state to reduce greenhouse gas emissions by 25 percent below 1990 levels by 2020. Cities and counties without a plan to reduce emissions within their jurisdictions risk losing transportation funding. With this in mind, two researchers from the University of California, Berkeley, set out to measure VMT per person in 370 urban areas around the United States and see how the built environment affects that number.

They discovered that people in more urbanized areas do drive less, but not that much less. Their lowest estimate for VMT was based on population density, but their more realistic result, which came from a model with more parameters, shows less of a reduction. Ironically, greater density is to blame for the lower than expected drop in VMT. As population density increases, VMT drops, but the densities of roads, shops, and services rise, all of which encourage automobile trips.

Urban areas need more roads because they have more traffic, which in turn tends to encourage more traffic. The researchers call it the “Los Angeles effect.” As Los Angeles filled the San Fernando Valley, it didn’t follow central travel corridors. Rather, it oozed out like a blob. This not only hindered mass transit in the city, it also required more roads to accommodate people’s varied travel patterns. The result is a “thicket of criss-crossing freeways and major arteries that form a dense road network,” the study’s authors write. Though Los Angeles is an extreme example of the road/population density relationship, other cities suffer from the malady, too.

The second curiosity that drives VMT in cities is the density of shops and services. It’s often easier to find a wider variety of goods and services closer to home in a city, which encourages people to leave home more often. So while people in cities still drive less, they don’t drive as little as they could.

Source:

Cervero, R., & Murakami, J. (2010). Effects of built environments on vehicle miles traveled: evidence from 370 US urbanized areas Environment and Planning A, 42 (2), 400-418 DOI: 10.1068/a4236

Photo by Josh Liba.

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Sears House No. 115

It’s cliché to say, “Everything that’s old is new again,” but boy if it isn’t true sometimes. I recently unearthed a monograph from 1942 about the conflict between urban and rural land uses, and a number of sections read like they were written yesterday.

George Wehrwein, the author of the monograph and a well respected land economist in his time, speaks with a voice that sounds distinctly modern. He assails unguided development as “suburban slums.” He points out farmland’s unfortunate role in absorbing willy-nilly growth. He mentions the more than 2,000 cities that, even in 1942, were utterly dependent on automobiles—“these cities have no street cars or buses of any type.” Even in that austere time, the beginnings of the automobile’s coming golden age were evident.

While the automobile and the “modern highway” were accelerating the pace of suburbanization, they didn’t start the trend—streetcars and interurban lines were initially responsible. “Almost as soon as railways became established, industries began to ‘decentralize’ by seeking locations in the suburban areas,” Wehrwein writes. While both trains and automobiles drove decentralization, they invaded rural spaces in distinctly different ways. Trains left a pattern of hub-and-spoke development, drawing some industries far out of the metropolis while leaving closer yet less accessible land under the plow. Automobiles allowed this decentralization to diffuse across the landscape even further while also invading the interstitial spaces left by train-focused development. “As a result, cities have not merely expanded, they have ‘exploded,’ ” he wrote.

Cars and highways spread development more evenly across the landscape, but much of the growth came at the expense of valuable farmland, something that clearly rankled the economist. Large tracts of land weren’t developed immediately, leaving empty lots set amidst trafficless roads, both of which became financial burdens on the local government. In his paper, Wehrwein condemns speculators that drove such slapdash development and rails against weak rural governments that did little to check them. He wasn’t universally panning the suburbs, but he was dismayed at what he saw as a waste of land and resources.

If Wehrwein’s lamentations sound distinctly modern, then so too do his solutions. He calls for large scale regional planning in his paper and advocates granting counties the power to guide development in unincorporated areas. Thanks to his earlier efforts, that experiment had already begun on in a few places. Twenty-five of Wisconsin’s 72 counties had zoning laws, and the state of California granted local authorities power to do the same. But Wehrwein also realized that granting authority does not ensure a desired outcome. “Mere power does not carry with it the desire, courage, or the wisdom necessary to make for a well planned rural-urban region.”

Source:

Wehrwein, G. (1942). The Rural-Urban Fringe Economic Geography, 18 (3) DOI: 10.2307/141123

Image in the public domain.

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