What do you want, mobility or access?

The third entry on the Gulf Coast Institute’s blog makes an important distinction:

…transportation researchers are supporting much more attention to “access,” or the ability to interact, as a different goal from “mobility,” or the ability to travel.

In Houston, we’ve always thought mainly about mobility. Here’s an HGAC map from 1989 (there’s a collection of them here) that illustrates the point nicely. The color coding represents speed (an earlier version of the Houston Real-Time Travel Map) and the grey lines represent travel time from Main and McKinney:


Traveltime Key

This map, in other words, measures how fast we can move through the city.

But mobility isn’t the real goal. I didn’t get up this morning and think, “I want to travel.” I woke up and thought, “I have to go to work.” I don’t care how far I can travel in how much time; I care about how long it takes me to get to my job.

Here’s a map by Transport for London that measures that (the original is on page 57 of this pdf). The brighter the color of the place you live, the more jobs you can get to in 45 minutes:


Employment Key

This map measures access. And access is what people want.

The ideal for a city isn’t how fast one can travel — rural Montana trumped every U.S. Metro area in that regard. The ideal is being able to get to a lot of places — jobs, colleges, stores, restaurants, theaters — in a short amount of time.

Every place has access zone around it — the places which you can get to in, say, 20 minutes. In some places, that zone is a lot bigger than it is in other places. But the key measure isn’t the size of that zone; it’s how much stuff is in that zone.

Houston’s actually doing pretty well by that measure. We’re low density — there’s less “stuff” per square mile — but high speed — so the zone is big. But it’s also possible to get the same effect by being low speed and high density — New York, for example. The ideal would be high speed and high density, but that’s pretty much impossible outside of sci-fi. The worst case is low density and low speed — Orange County comes to mind.

These are nice maps. But they’re more than that. What we measure plays a big role in shaping what we build. In Houston, we’ve been measuring mobility, and our biggest transportation projects — mainly expansion of existing freeways — are designed to improve that measure. In London, they’re measuring access, and they’ve been building projects — like improved road and transit links to new office developments in the Docklands — to improve access. We’re trying to speed up existing traffic flows, but we’re ignoring new ones, just like we ignored office growth in the Galleria 30 years ago (while we were busy drawing maps of travel time to Downtown) and have to live with the consequences now. If we think about transportation in terms of connecting people to where they want to go — not just in terms of moving cars or buses — we get better results.

Happily, HGAC has been drawing more useful maps recently — though still not as comprehensive as what TfL does. But are the policy makers looking? We should talk about access, and measure access and build for access. Access is opportunity and quality of life. Mobility is just speed.

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