Science of Cities

How Being Able to See Down City Streets Impacts Walkability

A look at the way “visual connectivity” affects pedestrian behavior.

A new study looked at Buffalo, New York, above, to consider how street design might impact walkability. (AP Photo/David Duprey, file)

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Amir Hajrasouliha first encountered the theory of “space syntax” — the idea that quantifying the geometry of an urban place can reveal something about how it’s used — as a master’s student in Tehran. It made sense to him, instinctively: Of course the design of a street network could impact land use.

The lessons of space syntax, formulated by the urban theorist Bill Hillier in 1980s Britain, lined up with his own experience in Iran. One of the main take-aways is that the visual continuity of street networks matters. Some streets — the ones that create longer vistas — attract more pedestrians, like Valiasr Avenue, the longest street in Tehran. Smaller streets, more segmented, encourage privacy and constrict pedestrian movement. In Tehran, villages that have been incorporated, over time, into the larger city exemplify this, with their tight, small streets that retain their intimate, residential character, even as the city grew around them.

When Hajrasouliha arrived in the U.S. to study for a second master’s at University of Michigan, he was surprised to see hardly any trace of space syntax research. (In fact, some of the theory’s most high-profile critics, including MIT’s Carlo Ratti, are based in the U.S.) One possible explanation for this omission, as Hajrasouliha saw it, was that the ideas of space syntax hadn’t been tested enough against real-world data.

So, when he had a chance, as a Ph.D. candidate in metropolitan planning, policy and design at the University of Utah, to pit the theory against an American city, he took it. The results are published in a new paper in Urban Studies.

With his co-author Li Yin, a professor at the University of Buffalo, Hajrasouliha looked at the impact of streets’ connectivity on the volume of pedestrians walking the streets of Buffalo, New York. A planned city, it has a semi-regular street grid with enough geometric variation to make it interesting to analyze.

Hajrasouliha and Yin looked at 302 of Buffalo’s street segments, on which grad students had collected years of data on pedestrian volume, and considered two ways in which the street design might impact walkability. The “physical connectivity” of each segment, a variable that planners often look at, was measured by the density of intersections in the surrounding area. The “visual connectivity,” a less often-used space syntax variable, was measured by the segments’ relative integration — essentially, how sharply angled each segment is to all others at a particular corner. And, when they crunched the numbers, they found that both the physical connectivity of the street and the visual connectivity of the street had a significant impact on pedestrian volume.

This map of Buffalo shows how streets compare in visual connectivity; the ones marked as “highest value” are the most integrated. (From “The impact of street network connectivity on pedestrian volume”)

To get a sense of what exactly this means, think for a second about a place like the West Village in New York, where there’s a dense network of streets but where you can’t easily see more than a couple of blocks ahead because there are too many odd angles. The shortest distance between two points might involve a complicated series of turns along a bunch of short little streets. And if you’re familiar with the place, you might take those. But there might be an easier route — you can go most of the way down a main street that cuts past smaller side streets, make just a couple of turns, and you’re there. Even if it’s not the most direct route, you might take this second option, where you can more easily see and understand where you’re going.

One of the criticisms of space syntax is that while it can be applied relatively easily to “organic,” unplanned cities like London, it doesn’t apply to the planned cities that dominate places like the U.S. Hajrasouliha and Yin’s paper doesn’t totally refute that criticism: While both the physical and visual connectivity impacted pedestrian volumes, the physical connectivity had a stronger effect.

“The geometric aspect didn’t show as strong correlation as it had in an organic city such as London,” Hajrasouliha says. “In those type of organic cities, [previous research] found a higher correlation.” One explanation is that planning did make a difference: Pre-determined land uses didn’t necessarily follow the natural logic of the street network. But the correlation between visual connectivity and pedestrian volume is there, too. “The geometry was significantly important, and it has it own impact on pedestrians,” Hajrasouliha says.

Hillier has argued for the primacy of space syntax — that a city’s geometry is the most important determinant of how people move through it. His critics rebut that this variable is less important than he’d like to believe — that analyzing streets’ geometry is “an exercise in abstraction,” as one put it.

Hajrasouliha’s work suggests that, to an extent, the critics of space syntax are right to question whether it has the same predictive power in a planned American city as in an organically grown European place. But his work also shows that the visual connectivity of streets is having an effect on how people use cities — that it’s not entirely abstract and that it shouldn’t be ignored.

“Increasing visibility and increasing physical accessibility will definitely increase walkability of the space,” he says. “When you’re a designer who wants to design a big area, don’t just think of accessibility in terms of physical distance. The visual connection can be a factor in inviting people to move around.”

It’s another variable for planners and designers to play with: The best city streets may not simply get people from point A to point B quickly, but let a person standing in one place see the possibilities of another.

The Science of Cities column is made possible with the support of the John D. and Catherine T. MacArthur Foundation.

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Sarah Laskow is a reporter and editor in New York who writes about the environment, energy, cities, food and much more.

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Tags: urban planningwalkabilitypedestrian safety

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