I keep stumbling across a great transportation visualization project from the Social Computing Group at the MIT Media Lab, most recently in this Washington City Paper post. In a series of interactive maps, covering a dozen cities, the Media Lab has mapped the most efficient mode of transportation — by car, bike, foot or transit — between any two points in a city.

This is what such a map looks like in Washington, D.C., if, say, you're beginning your trip from Capitol Hill, inside the green block:

Leaving from that part of town, more than half of Washington is reached faster by bike (yellow) than any other mode of transportation. The same is true of less than 1 percent of the city by transit (in blue). Here's the lab's method of figuring this out:

To make this map, we gridded up the city at the block-group level, and then computed the time using each mode of transport from the centroid of the source block group to the centroid of the destination block group using the Google Maps API. For driving, we added a buffer time for parking and walking, and then we compared the four resulting times and colored the block-group based on the minimum.

As a tool for planning your travel routes, or even picking a neighborhood to live in, this is a fascinating platform (albeit a limited one: yes, it doesn't touch on comparative costs, the existence of bike lanes, or the impact of road congestion at different times of day). But beyond personal applications, this type of map has some policy implications, too.

Two things are particularly striking about the above picture: Cycling is a much more efficient mode of transportation than many people realize. And transit is startlingly not so. Seldom will it get you farther, faster, than a bike will. Here's a picture of your transit prospects from the other side of the Anacostia:

Very little of the city — just one tiny patch of it — is accessed fastest by transit. This picture would no doubt look different if we removed bikes from the calculation entirely and simply compared cars and transit. But even then, the city would still look more broadly accessible to you from behind the wheel of a car. The same is true even if you live on a transit line:

Look across the other cities in the collection, and transit appears equally inefficient, relative to both cars and bikes. Here's a sample from Philadelphia:

From Chicago:

From San Francisco:

You can look at these maps and conclude that more people would be better off biking, and that cities should invest in the infrastructure that encourages them to do so. This is true. But, when about half of one percent of all commuters nationwide currently cycle to work, it's probably unrealistic to expect that most people in those yellow blotches will regularly travel that way.

Another takeaway is that these maps illustrate why people make rational calculations to drive so much of the time, even in cities where decent transit does exist. The total financial cost per trip of driving somewhere is likely higher than taking transit (or biking), once you factor in car payments, insurance, and maintenance. But we tend to treat those as sunk costs. And so we often make travel decisions with a time budget in mind, not a financial one. By that metric, it's clear here why people who can afford to drive often chose to. It's also clear on these maps that people who can't afford a car pay a steep penalty in time to get around.

Transit advocates spend a lot of time worrying about the lack of appeal of transit for "choice riders," or commuters who have other options for getting around. It's important to recognize that the decisions they make are often weighed in time.

That means that a big part of the challenge here for cities is to make transit a more efficient travel mode, relative to cars, for more people. The balance between the two options is notably different in Manhattan:

But outside of New York — with its extensive subway system — this is an extremely difficult task, particularly given that most of these maps reflect the fact that we've built cities to be traveled by cars (by, for instance, routing highways through them). But it's possible to increase the relative efficiency of transit by creating dedicated lanes and signal priority for buses at stoplights, or increasing forms of express transit service. Transit networks could even compress what feels like the time cost of riding transit by adding cell service and WiFi that enable passengers to use time spent commuting productively — and in ways that aren't possible from the driver's seat of a car.

It's also possible to increase the number of people for whom transit is an efficient option if we add housing and offices to those parts of any city where transit already exists. More density wouldn't change the shape of these maps. But it would change the number of people who stand to benefit from the parts of the map that are blue.