Mobility vs Accessibility: new evidence for an old debate

I was at a public exhibition many years ago where I was approached by a rather aggressive environmental campaigner. He told me that if public sector transport planners like me really wanted to promote sustainable travel, then we’d all live and work within the same Local Authority area. Everything we did should be local and we shouldn’t really need to go anywhere, and then we wouldn’t be emitting all that carbon travelling. As I lived in another District from where I was working, albeit only 10-12 miles away, I naturally did no more than nod and smile politely.

Inside, I was thinking: Yes I agree that we need to reduce our transport emissions and impacts on the climate, but my husband is the one that lives next to his work and I had to find a job nearby. Yes I’d like a shorter, more convenient commute, but there wasn’t a job in my field, never mind at the level I was looking for, advertised within my District at the time. Yes I prefer to travel by sustainable modes, but I do take the train to get here, whereas I might have to drive to other jobs at a similar or shorter distance. Yes, but…!

Ok, enough of the protests in my head that clearly have been yearning to break free for far too long. My point in recalling this story is that the man’s superficially inane, impractical argument does have a grounding in a fundamental principle of transport that many transport planners, never mind transport users, often overlook. Transport planners tend to focus on creating and promoting options (read new infrastructure or services) for mobility, rather than accessibility.

Yet people travel for the purpose of accessing a job or a shop or a friend’s house, and travel further if those things they are trying to access are further away. The further they travel, the less mobility options they have, which may result in a poor choice between car-dependence and isolation. The latter I add as we consider the impact of new online technologies on accessibility over mobility. See a great blog on this by @alikirkbride for #LTTMobilityMatters.

Moreover, I have recently discovered that the concept that humans seek accessibility rather than mobility can be backed up scientifically. In the last decade, researchers [1-4] have used big data from mobile phone call records and social media to show that human movement follows certain patterns, namely:

  • Most people can be found in a few predictable places (home, work) most days of the week at the times (night and day) where you’d expect to find them there.
  • Most people make more short trips than long trips, and the distribution of short trips follows a certain pattern, decreasing with distance, up to a threshold.
  • At which point you have a different pattern where people who travel further can be found in expected places more often and have fewer irregular trips.
  • And, those people who travel further tend to live where there is less density – of population, employment, opportunities, activities – than those who travel shorter distances.

It is this last point that is key. None of the studies are looking at mode of travel, but they say something very basic about travel behaviour. Namely, that people are not choosing which trips to make to minimise journey times or distance travelled, even if that may influence modal choice. No, they are choosing which trips to make based on where the destinations are which they are trying to reach. They will choose the nearest destination that meets their need or desire or nearest ‘intervening opportunity’ as one study calls it [2].

Thus, transport planners should be as aware as land use planners of the importance of place-making, of mixed-use development, of walkable neighbourhoods. Discussing those is a whole other blog, so I’ll leave it there, but in a twisted way, that man who chastised me long ago had a point. If we could work at the sort of job we wanted, shop for whatever we needed, socialise with our friends and family and have our children in decent schools, all in the same area as our home, we would probably choose to do so. Then we would have more options for sustainable mobility (e.g. walking and cycling), which would be better for the environment and our health and make us more resilient to unforeseen events. And so transport planners would be planning for accessibility rather than mobility.

  1. Gonzalez, M.C.H., Cesar A. & Barabasi, Albert-Laszlo, Understanding individual human mobility patterns. Nature, 2008. 453(7196): p. 779-782.
  2. Noulas A, S.S., Lambiotte R, Pontil M, and Mascolo C, A Tale of Many Cities: Universal Patterns in Human Urban Mobility. PLoS ONE, 2012. 7(5): p. 1-10.
  3. Isaacman S, B.R., Caceres R, Kobourov SG, Martonosi M, Rowland, J and Varshavsky, A. Identifying Important Places in People’s Lives from Cellular Network Data. in 9th International Conference on Pervasive Computing (Pervasive). 2011.
  4. Song, C.Q., Zehui Qu; Blumm, Nicholas and Barabási, Albert-László, Limits of Predictability in Human Mobility. Science, 2010. 327: p. 1018-1021.

 

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