– David Levinson

Dennis from Axistive.comod point.

Maybe a better way of phrasing the issue would be “mobility for cars alone” vs. “mobility for everyone.”

Imagine a hypothetical auto-utopia (or maybe dystopia), where people could get anywhere within half an hour by driving 70 mph but where there were no sidewalks or public transit, and every street was twelve lanes wide. Drivers would have high levels of mobility. But people who (for one reason or another) did not drive would have zero mobility.

Query: is this the society we want? And if not, what are we going to do about it?

The perspective is a necessary outcome of the consensus view that the demand for transportation is derived, meaning that people do not usually travel for the pleasure of motion per se, but in order to reach destinations. The derived nature of travel demand implies that accessibility is the service that is provided by mobility; that is, access is the ends and mobility is a means. Yet there are other routes to access as well, including proximity and remote connectivity.

The question is how we evaluate transportation-policy outcomes. If we evaluate them in terms of either mobility, proximity, or remote connectivity, we are measuring means, or inputs. It would be like developing a standard for illumination in watts rather than lumens. “Derived demand” implies that we ought to be measuring accessibility—the desired service provided by transportation—to gauge transportation outcomes. To see why this matters, imagine a hypothetical set of transportation policies and investments that, in aggregate, improve mobility but degrade accessibility. (This would happen if they induce land uses to move farther and farther apart, and the added distance isn’t fully compensated with increased speed). These policies would leave people with less time and money with which to interact with their destinations. If the demand for transportation is derived from the desire to reach destinations, this must be seen as an undesirable transportation-policy outcome.

So taking “derived demand” seriously implies reordering the goals hierarchy and putting accessibility on the top. The question then becomes, “accessibility to what,” which I take to be at the heart of Randy’s concerns. If we knew individuals’ wants, needs, and preferences, we could presumably estimate their accessibility in a given land-use and transportation environment. In absence of that knowledge, we can use access to a range of choices as an approximator. You may like Oaxacan restaurants and hate Canadian restaurants (lucky you live in LA and not Ann Arbor), but we don’t know that, and as a consequence measure access to retail and services generally. There’s a reason behind it: the larger the number of goods and services you can reach with a given investment of time and money, the more likely it is that you can get what you want.

One hopes that we’ll get better at measuring accessibility, based on improved data and methods. Accessibility measurements are surely an approximation, as opposed to mobility measurements, which can be very precise. But the derived nature of transportation demand means that as a policy yardstick, accessibility is approximately right, and mobility is precisely wrong.

Jonathan Levine

References

Kenworthy, Jeff and Felix Laube (2002). Urban Transport Patterns in a Global Sample of Cites and Their Linkages to Transport Infrastructure, Land Use, Economics, and Environment. World Transport Policy and Practice, 8(3), 5-19.

Thomson, J. Michael. (1977) Great Cities and their Traffic. London: Gollancz.

The real problem is that the SmUGLERs employ dishonest assertions. They conflate the fact that the current POV/roads model is not perfect as being one of fatally flawed. At the same time they promote alternatives as if they carried no externalities whatsoever.

The supply of destinations has value which is capitalized in land. There is a value to opportunities or accessibility, which should also be plain.

The demand for travel to a place is of course a function of opportunities there, elsewhere, and the respective costs, as well as characteristics of the individual. The demand for development in a place also strongly depends on existing accessiblity.

I would hardly presume to speak for all researchers, but in Minnesota we have been trying to be very clear that accessibility is a measure of ease of movement to opportunities (which is far better than mobility, which is simply a measure of ease of movement without any relationship to where that movement is … movement to opportunities is more important than movement between two vacant parcels).

The transportation profession (engineers at DOTs) has been moving very much towards performance measures or performance indicators, as they are solving a multi-objective problem (welfare is far too complex to define, much less optimize, when you are dealing both with clearing snow, filling potholes, straightening roads, and investing in new highways). Books like The Balanced Scorecard have been influential. Unfortunately, mobility has been used as a measure to the exclusion of accessibility.

Accessibility can be an important performance measure/indicator, and an important output to compare alternative investment scenarios (does investment A or B improve accessibility (to jobs, to shops, etc.) more?).

Ceteris paribus, more access is better than less (arguments like Schwartz’s Paradox of Choice aside.

– dml

Implicitly including demand (i.e., the value of access) but not always recognizing this, explains much of the considerable confusion over these measures and their usefulness for policy. I suggest we say so plainly.

Rand