How transit access changes the perception of distance and accessibility of resources in a city.

By now, tales of Hurricane Sandy’s impact on the New York City transit system have been widely and well told. The storm managed, with awe-inspiring force, to sever the subway arteries of the largest city in America. And just as impressively, New York managed to bail out the water and get moving again.

But we were struck recently by a pair of novel images of the city before and after its transportation network fell into disarray. These two maps come courtesy of the developers behind OpenTripPlanner. This first one, showing the city before the storm, illustrates how accessible each corner of New York was with a typical morning’s use of the transit system:

OpenTripPlanner Analyst project

The yellow areas are the parts of the city that 7.5 million New Yorkers can reach from home in less than an hour by public transit and walking. The red areas are within an hour’s commute of 6 million people in the metropolitan area. The blue areas are accessible by 4 million people, and the gray areas by 2 million.

And then there’s this map, which shows the same landscape as it was accessible to New Yorkers right after Hurricane Sandy:

OpenTripPlanner Analyst project

To unpack exactly what we’re looking at here – and what these pictures mean for urban planners elsewhere – it’s helpful to step back a bit. Geographers and planners have talked for decades of building accessibility measurements to illustrate and understand how far people are located in cities from the resources they need. But until now, it’s been difficult to paint that picture with any real fine-grained detail. You may have seen, for instance, more basic maps that illustrate all the jobs or affordable housing units within a given radius drawn around a metro station.

“You’d see low levels of detail, and big blocks of color,” says Andrew Byrd, one of OpenTripPlanner's main contributors. He's been trying to re-purpose the tool for transportation planning and analysis and worked on the above maps. “We’re trying to get very precise, and we’ve been able to do that by having all this data available that we didn’t have before.”

These two maps were built using Census data and the OpenTripPlanner route-planning tool, a multi-modal, open-source platform that models travel itineraries for users (with the help of GTFS data feeds from transit agencies, among other sources). Sandy effectively disabled all of the underwater subway tunnels connecting Manhattan with other parts of New York, dramatically altering the transit service for several days. When that happened, New Yorker Alastair Coote modified the public GTFS feed from the New York Metropolitan Transit Authority to remove the closed subway stations and account for added shuttle service after the storm. By doing that, he created an emergency route-planning tool using OpenTripPlanner to help New Yorkers navigate the city with dramatically reduced transit service.

Byrd had meanwhile been experimenting with OpenTripPlanner as an instrument for urban-planning analysis. "This was kind of a perfect situation to apply it," he says. Coote's modified GTFS feeds created an opportunity to map mobility in the city in scenarios with normal and dramatically altered transit service. You can think of the above images as insanely complex travel-time maps. Cities previously wrote about travel-time maps like this one, which illustrates how far a person can travel on transit in 30 minutes from the center of Chicago:


Byrd's maps more or less simultaneously illustrate that picture, before and after Sandy, for every person living in the New York Metropolitan area (based on Census data showing the number of residents per block throughout the city). “It’s as if everyone in the city took transportation at the same time to everywhere they could possibility go – at the same time – and then you accumulate everything,” Byrd explains.

You can also think of the result this way: For each pixel on the above maps, OpenTripPlanner has calculated how many people in New York are within an hour of there by transit and foot. Manhattan normally appears as the most accessible place in the city, shown in bright yellow in the first map. And this is no accident.

“There’s really this incredible connection between how accessible places are, and how densely populated they are, and densely used and densely built up they are,” Byrd says. You can argue about whether this correlation happens because transit access enables density, or because transit follows density where it already exists. “To me the most important thing is not which one causes the other,” Byrd says. “The most important thing is that public transit networks – the structure they have, and how they’re connecting things – that completely changes how people perceive distances.”

In cities, we perceive distance not by miles traveled but by the time required to cover those miles. And so bright yellow Manhattan feels like the center of New York, even thought it isn’t geographically in the middle. “It’s the place where the most people are the closest to the most other people,” Byrd says.

At least, that was the case the day before Sandy.

“Just cutting some of the lines in the transit system completely changes that landscape,” Byrd says.

This third map shows the difference between the above two – in other words, the places where accessibility was most dramatically impacted by Sandy:

OpenTripPlanner Analyst project

The city has long since returned to normal, to the map shown at the top of this post. But looking back on these visualizations, two things become clear for New York and any other city moving forward: Transit access dramatically changes a city's landscape as it's experienced by people moving through it, and unlikely tools like trip planners may be just what we need to start filling out that picture.

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