Counting intersections

By Felix Salmon
September 19, 2011
remember her from her great article on slugging in DC -- has a fantastic post on street-map design over at The Atlantic Cities.

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Emily Badger — you might remember her from her great article on slugging in DC — has a fantastic post on street-map design over at The Atlantic Cities.

Garrick and Marshall’s research into street network patterns began in Davis, California. Often cited as the most bike-friendly city in America, it has the country’s highest rate – more than 16 percent – of people commuting to work on two wheels. It turns out, though, that Davis also has one of the lowest traffic fatality rates in the country, a counterintuitive discovery for traffic engineers who consider biking a riskier alternative to driving.

Inspired by Davis, Garrick and Marshall compiled data on 230,000 crashes spanning 11 years in 24 medium-sized California cities. And they began to parse and classify street patterns in a kind of taxonomy. There are networks that look like square grids and others that resemble trees, with one trunk, many branches. There are networks that have tributaries, like a river, and others that have main roads radiating out from a central hub. There are hybrids of all these, and street blocks of different lengths, and networks that have 45 intersections per square mile (like Salt Lake City) and others that have as many as 550 (Portland, Ore).

In their California study, Garrick and Marshall eventually realized the safest cities had an element in common: They were all incorporated before 1930. Something about the way they were designed made them safer. The key wasn’t necessarily that large numbers of bikers produced safer cities, but that the design elements of cities that encouraged people to bike in places like Davis were the same ones that were yielding fewer traffic fatalities.

These cities were built the old way: along those monotonous grids.

FHA 3_.jpgWhen streets are built to a human scale, rather than being built for cars, those streets are friendlier and safer. More generally, the metric of intersections per square mile is an incredibly useful idea to keep in mind. It’s correlated with density, but it’s not the same thing at all. For instance, Badger reproduces these maps from the Federal Housing Authority, back in the 1930s: we’re seeing two plans, here, with identical housing density. But the one labeled BAD has ten intersections, while the one labeled GOOD has only seven. And of course the distance you need to travel to get from any random point to a given house is much shorter, on average, in the BAD map than it is in the GOOD one.

Shorter distances mean that you’re more likely to walk or bike; they also make the neighborhood feel smaller. For many decades, suburbia was designed on the idea that Americans want to feel as though they’re far away from each other, but the fact is that we need community and linkages just as much as we need a space of our own.

A city with high density still feels inhuman if it has enormous blocks; it doesn’t matter how many people you squeeze into downtown Phoenix, it will never feel like a vibrant, high-density city. Even in New York, the distance between 5th Avenue and 6th Avenue is far too big: the shorter blocks east of 5th Avenue, between, say, 5th Avenue and Madison Avenue, are much more pleasant, as are the small blocks in the Financial District.

It’s astonishing to me that Portland and Salt Lake City can differ, in terms of intersections per square mile, by a factor of more than 12. According to Wikipedia, Salt Lake City has a density of 1,666 people per square mile, compared to Portland’s 4,288. Which means that Portland has almost five times as many intersections per person as Salt Lake City does.

Which raises another question. Do street intersections make you liberal?


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Great post. Reminds me of what Jane Jacobs said.

Posted by jmh530 | Report as abusive

This is why I read you, Felix. I learn something new almost every day.

That said, while the underlying research may be interesting, I was disappointed that the article lacked any sort of rigor to its argument. In general, there are too many confounding variables to draw the conclusions that it does. For example, while it’s certainly possible to believe that people drive more in cul-de-sacs than in traditional blocks, it’s not at all clear that it’s due to the street design as it is when the neighborhoods were created.

The relative locations of grocery stores and offices don’t at all seem to be functions of the street layout. It almost seems like the author is intentionally confusing the purpose of cities and suburbs.

In short, the article started by assuming its conclusion, and declined to look at other explanations for the data. While the conclusion may be true, this decidedly not fantastic article does it a real disservice.

Posted by Curmudgeon | Report as abusive

The distance between 5th and 6th Ave is “far too big” ? Really Felix it’s hardly a howling wilderness between those two avenues, and the rabbit warren aspect of some narrow streets downtown is really not charming. (Pearl & Pine St area). The side streets between 5th and Park – short blocks bisected by Madison Ave – are also cramped and dreary. Similarly 38th Street between 7th and 8th Ave isn’t particularly pleasant – but in all these cases thats mainly the fault of the buildings on either side of the street. Walk down West 22nd St. between 9th and 10th and the “far too big” distance is much nicer than many shorter blocks.

Posted by wah718 | Report as abusive

I recently moved from the suburbs of Houston to near the center of the city. Here’s a map of my old neighborhood, and here’s a map of my new neighborhood.

I now have numerous destinations within walking distance, and many more within biking distance. Previously, destinations within walking distance were few.

I love my new neighborhood. Guess which one was built before 1930.

Posted by BruceHeerssen | Report as abusive

Hmmm… How do fatalities in the Village or financial district compare to those in Hell’s Kitchen, where i hang my harness?

Posted by samadamsthedog | Report as abusive