Accessibility vs Mobility

Accessibility and mobility are two quite interesting terms that get thrown around a lot when it comes to transport planning. Humantransit has an excellent post quite recently on the pitfalls of trying to analyse public transport projects as if they were roading projects. The post outlines that one of the pitfalls is the issue of frequency of public transport services, to which there is no real counterpart in roading projects – meaning that its importance is generally under-rated by transport planners who come from a roads-based background. The other issue is the differing opinion on what “accessibility” means, how it interacts with the concept of “mobility”, and how that all affects the way we approach transport projects.

Traditionally, road-based transport planners view accessibility and mobility being in natural conflict with each other – by that I mean the more accessible a road is the lower the speeds along it are, and therefore the less mobility one has. A classic diagram from the AASHTO Green Book (some sort of roading bible I assume) shows this ‘conflict’:

In short, as access gets higher, mobility reduces – and the vice versa. This type of analysis has led to the “hierarchy” of roads that we generally have, to provide all of the different options – so that access is provided where needed, and mobility is provided where needed. That’s smart in the sense that you wouldn’t exactly want driveways backing onto motorways.

In terms of applying this concept to public transport, I suppose in some respects different types of public transport could be seen as providing different levels of access and different levels of mobility. A local bus would have great access but is unlikely to be particularly quick – especially if it meanders around local streets like many in Auckland do. Similarly, a High-Speed Railway line would provide tremendous mobility, but is likely to have poor access – as generally a city would have only one or two High-Speed Rail Stations to ensure trains don’t get slowed down too much by having to stop (although this nifty idea might in the future fix that problem). Just as you have minor arterial routes, major arterials and motorways, a good public transport network (as proposed, but generally not yet implemented, by ARTA) will have local services (high accessibility, lower mobility), ‘quality transit’ (medium accessibility, medium mobility) and ‘rapid transit’ (lower accessibility, higher mobility). Furthermore, just as one drives from a minor arterial onto the motorway, one would transfer from a local feeder bus onto a rapid rail system.

So there are certainly plenty of similarities to the concepts of accessibility and mobility if you look at the two matters from this perspective which, critically only focuses on transport. However, what’s really interesting is when you start putting together this transport perspective with the way our urban form is structured. The Humantransit blog post picks up on how things get complicated once you start throwing land-use patters and urban form into the mix:

…when interacting with highway engineering, it’s crucial to notice that the highway engineering meaning of the terms access (or accessibility) and mobility (or movement) is quite different to the way those words are used in sustainable transport planning and urbanist thinking. The terms access and movement in the … diagram above refer exclusively to the tradeoff as it affects vehicles. If everything that matters is a vehicle, then access and mobility do come into conflict, and the different standard road types, from freeway to cul-de-sac, represent different points of balance between them, as the diagram indicates.

But when sustainable transport people talk about mobility and access, they’re extending the concept to apply to all humans rather than vehicles, which makes the issue vastly more complex and multidimensional. They’re also transforming access into something more purposeful, and assigning it a moral weight that it doesn’t have in the highway world. In the sustainable transport formulation… mobility is how far you can go, while access is how many desirable things you can do. You can improve your access, but not your mobility, by moving closer to work, or moving in with your romantic partner. In this urbanist formulation, access can be improved through by putting desirable things closer together — a process that we all consider when we decide where to live, and which urban designers do in the aggregate when they design or redesign communities in response to the demands that our individual decisions have generated. For that reason, access… has taken on a moral superiority over mobility which it doesn’t have in highway thinking.

So be careful with mobility or movement and access or accessibility. Urbanists and sustainable transport people mean one thing. Highway engineers mean something much simpler — so simple in fact that you can make a diagram of it.

If I am looking at what I might hope from a transport project, in the end I think that it is increased accessibility – namely that it is easier (encompassing quicker, cheaper, more convenient, more reliable etc.) to do what I want to do than it was before. As was briefly mentioned in the book review I blogged about recently, Manhattan could be considered the most accessible place on earth – as just about anywhere you live on that island you can probably find most of anything you’ll need within a short walk from where you are. However, from a mobility point of view all one would ever hear about is how congested the streets are and how long it takes to get from one place to another. My point being “who cares, if all I need is nearby?”

Interestingly, if we look at the interaction between mobility and accessibility from a place based perspective, by which I mean analysing the level of mobility a place has compared to the accessibility it has, we may come across the same strange conflict: that as it becomes easier to go further, stuff generally becomes more spread out. As mobility improves, what is within relatively easy walking distance (my somewhat arbitrary definition of accessibility) will lessen.

All of this becomes particularly fascinating when we look at roading projects, and the benefits they are supposed to provide. Roading projects are generally predominantly justified on the basis of their time savings benefits, basically the amount of increased mobility the project will provide. Instead of only being able to drive 10km in 15 minutes, with this new project you’ll be able to drive 20 km in that same 15 minutes – that’s a huge benefit (supposedly)! Generally projects are not quite analysed in that way – but rather along the lines that it will take people x fewer minutes to travel a certain distance, but I think my way of looking at things is more realistic, because history tends to indicate that if people can travel 20km in that 15 minutes, rather than 10km, then they will take advantage of the increase in mobility and start spreading out more. This leads to the level of accessibility either remaining the same (if you take accessibility to potentially mean “what can I do within a 10 minute trip of home?”) or decreasing (if you take accessibility to mean “what can I do within a 10 minute walk from home?”)

So I would recommend that we start thinking about accessibility more, in a general urbanist sense, when we’re looking at transport projects – rather than just mobility. I tend to think of increased mobility as potentially the means to the ends, that hopefully in some situations if we improve the speed at which we can travel we will increase what can be reached within a certain space of time. (Of course the interesting matter of induced demand tends to undermine that approach). Surely what we really want out of a transportation project is something that will help us achieve more, help make it easier to get to places, help reduce the constraints that travelling around places on what else we can do with our lives. To me these matters are all about what increased accessibility provides – and, critically, it is as much about urban form and land-use planning as it is about transportation. You simply can’t do one without the other (are you listening SJTA?.)

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