Donald Rumsfeld’s lasting contribution to modern thought is his statement that “there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don’t know we don't know.”
It sounds silly, but it’s actually a useful way to break down complicated and ambiguous situations – much like the recent decline in driving in the U.S. and several industrialized countries, the topic of an International Transport Forum (ITF) roundtable I was privileged to attend last week in Paris.
First, for the “known knowns.” We know that aggregate levels of driving in several industrialized countries (including the U.S., which we’ve blogged about here, here, here, here , here and here) started to stabilize sometime between the mid-1990s and mid-2000s. In a few countries (again including the U.S.), that stabilization was followed by a mild drop in driving after 2007 or so. That represents a dramatic change in trajectory from the trend of the previous several decades of regular, year-over-year increases in driving.
The chart below, from the ITF’s 2012 Transport Outlook, shows the similarities in trend among the countries.
Another “known known” is that these countries have had a few things in common: all have been experiencing economic challenges and all have aging populations. Both of these developments would be expected to lead to reduced per-capita driving, all other things being equal.
Finally, we know that the trend toward reduced driving has been led in many countries by young people. We documented the U.S. version of this trend in our April 2012 report, Transportation and the New Generation.
It is the startling magnitude of the decline in driving among young people, however, that adds the greatest number of “known unknowns” to the mix. There are good reasons, for example, to expect that delays in household formation (driven either by the economy or long-term trends toward later marriage and child-bearing), as well as tighter criteria and higher costs for drivers’ licenses, could cause young people to delay car ownership or acquisition of drivers’ licenses. Perhaps, some suggest, today’s young people will turn out to be just as willing to drive as preceding generations, but they’re just waiting longer to get started. It remains too early to tell for sure.
In addition, young people and others are increasingly seeking to live in walkable neighborhoods, often in close proximity to city centers – a trend which we’ve discussed (here) in the U.S., but that appears to be common to many western countries. Are these desires likely to persist – both over time and as individual young people age? And, if so, how will those desires be accommodated in a country where the supply of affordable, quality urban housing is already in short supply?
Last, but possibly most importantly, is the question of how the mobile technology is reshaping our lives will impact people’s use of transportation. It’s very likely (at least to me) that these changes are going to be significant, but it is way too early to assess their magnitude.
This is already a daunting list of questions to answer, but it’s doesn’t even begin to touch on the “unknown unknowns” – the technological innovations and societal changes that are in store for us, but that we haven’t even begun to consider. In times past, the historic stability of the trend toward increased driving – as dependable, to paraphrase my colleague Phineas Baxandall, as the sun coming up in the morning – provided the sense that, whatever the unknowns, we could feel confident in making the long-range projections that underpin transportation planning. Those days are clearly gone.
We’ll have much more on some of the exciting research presented at the roundtable on driving trends in the days and weeks ahead, as well as more details on our new work on the implications of shifting driving trends on transportation policy shortly after the new year.