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The surprising ways size and shape can impact a place's economic productivity and walkability.

Cities often celebrate the anniversaries of major pieces of transformative infrastructure, like bridges or buildings or dams. It's much more rare to celebrate the birthday of a design template. The bicentennial of Manhattan's street grid, which fell in 2011, was an exception. There was an exhibit at the Museum of the City of New York to mark the milestone. Countless articles from planners, architecture critics, and urbanists lauded the foresight of the city's street commissioners, who in 1811 laid down the plan that defines the island's development to this day.

On the occasion, the New York Times architecture critic Michael Kimmelman, wrote this about the "oddly beautiful" grid:

It's true that Manhattan lacks the elegant squares, axial boulevards and civic monuments around which other cities designed their public spaces. But it has evolved a public realm of streets and sidewalks that creates urban theater on the grandest level. No two blocks are ever precisely the same because the grid indulges variety, building to building, street to street.

New York, of course, is not the only city built on a grid. Similar schemes could be found as far back as ancient Greece and Rome. But Manhattan's design was the exemplar for what became the default pattern of American cities.

Still, not all grids are created equal. Some shape a walking-friendly streetscape. Others, not so much. Over at the Strong Towns blog, Andrew Price, a software developer by day who blogs about urbanism, has been writing about the math of the grid and what it reveals about a city's economic productivity and walkability.

Price has created a "street area calculator," that allows you to plug in a street width and block size. Using this tool, you can come up with some basic figures to compare different grids and how they apportion a city's land. To take two of the extreme examples calculated by Price using rough figures gleaned from Google maps, Portland, Oregon, has streets that are 60 feet wide (building face to building face, including the sidewalk) and blocks that are 200 by 200. Compare that to Salt Lake City, where the streets are 130 feet wide and the block are 660 by 660.

Portland, Oregon (left) and Salt Lake City, Utah.

These configurations mean that Salt Lake is using its space more efficiently by one measure, with only 30.2 percent of area devoted to streets, which must be maintained and are not "productive" in terms of tax revenue. Portland, in contrast devotes nearly 41 percent of its area to streets. Most street space goes to cars, with sidewalks taking up a relatively small fraction.

But when you look at how much street frontage a city’s grid creates within a half-mile walk of a certain point – one potential measure of walkability – Portland has nearly 160,000 feet, while Salt Lake has just under 60,000.

Price points out that if you create smaller blocks, more space goes to streets (and usually, in this country, that means it goes to cars), and the width of the street must be adjusted in order to create a pedestrian-friendly environment:

If we are to downscale our blocks to make our grid more walkable, we also need to downscale our streets, in order to keep the ratio of Street Area:Block Area down. We can have 150 ft blocks and keep our street area down to 22.15%, if we also build 20 ft streets (which would result in 284,800 ft of street frontage being within a half a mile walk - far greater than that of Portland!) 
However, when we start talking about 150 ft blocks and 20 ft streets, we begin to get into the realm of traditional cities.

Traditional cities are naturally highly walkable, human-scale environments. 

Price’s work is inspired in part by the disorientation he felt upon moving to the southern United States from a more "human-scale" community. "I was born and raised in Australia, in a middle-class inner-city neighborhood," he wrote in an email. "I grew up around walking, transit, and street life. Two years ago, I relocated ... From dealing with the culture shock (most towns are simply a road with a couple of strip malls and drive through, very few actual 'urban' places where you can make a day of walking around), I've turned to blogging as a way to study and cope with the lifestyle change."

In most cities with wide streets and big blocks, Price says, precious little space is allotted to pedestrians. According to his calculations, 30 percent of a city’s area is typically dedicated to moving cars – "not counting the parking lots that push some southern cities over 50 percent."

Price hopes that by examining the proportions of the grid from a mathematical perspective, we can better understand what makes some grids a better place for humans to live than others.

Top image: Chicago's street grid. Scott David Patterson /Shutterstock.com

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