Elements of Access: Constant Travel Time Budgets?

By Kay Axhausen

Travel time [h] in England and Wales since 1970. Source: Metz (2008) p. 323
Travel time [h] in England and Wales since 1970. Source: Metz (2008) p. 323
One of the most famous claims made about travel behavior is that the time spent on it is constant over the years (Zahavi, 1974). This a claim made for whole populations not individuals, where personal introspection and observation tells us, that the time spent changes with age, family responsibilities, new workplaces or homes. It is a claim made for regular daily travel, i.e. that happening in the usual environment of the traveller. See the Figure for an example of this observation.

Zahavi (1974), and all those in his tradition, base this claim on a striking similarity of the reported numbers for total daily travel time in local, regional and national travel diary surveys (e.g. Schäfer and Victor, 2000 or Metz, 2008). Originally a figure of 60 min was proposed, but this has crept up over the years; About 100 daily minutes are reported in Switzerland for example.

The claim is powerful, when linked to Downs’ (“law of peak-hour-expressway congestion” or triple convergence (discussed in Downs, 2004), which observes correctly that travellers will respond to changes in the transport system by changing their behavior until they cannot find a way to improve their situation further. The changes can be caused by travellers improving their daily schedule (leaving or arriving earlier), by them changing to a more attractive mode, by them switching the work place or the residential location for something better or by them doing things more often out-of-home: meeting friends, attending civic meetings or coming along to one’s child sport events.

The investment in the transport system will have generated first travel time savings, but in longer term these were converted into other things: the seeming paradox or law of peak hour congestion. You can say the investment was in vain, when you look only at time use or motorway speeds, and argue that all investment is pointless (e.g. Metz, 2008, although he is not quite as radical as the political discussion), or you can ask, if these long term changes aren’t indicators that the population can now live better by using the new capacity to do things they want to do when, where and how they want to do them.

Next to political assessment is the empirical question, if all of the change is converted into longer travel. If taken seriously the constant travel time budget implies an elasticity of minus one of travel distance with respect to changes in travel time (speed). Only then can  the budget stay constant.

There is some work on this in the context of the ‘induced demand’ discussion (e.g. Cervero and Hansen, 2002 or Weis and Axhausen, 2009), but not nearly as much, as the size of the claim would justify. All of the existing work indicates that the elasticity is large, but not nearly one. This tells us, that some of the gains remain in the transport system, mostly as better daily schedules for travellers.

References:

Cervero, R. and M. Hansen (2002) Induced travel demand and induced road investment, A simultaneous equation analysis, Journal of Transport and Policy, 36 (3) 469-490.

Downs, A. (2004) Still Stuck in Traffic, Brookings Institution, Washington D.C.

Metz, D. (2008) The myth of travel time saving, Transport Reviews, 28 (3) 321-336.

Bundesamt für Statistik and Bundesamt für Raumentwicklung (2012), Mobilität in der Schweiz, Ergebnisse des Mikrozensus Mobilität und Verkehr 2010, Neuchâtel and Bern

Schäfer, A. and D.G. Victor (2000) The future mobility of the world population, Transportation Research A, 34 (3) 171-205.

Weis, C. and K.W. Axhausen (2009) Induced travel demand: Evidence from a pseudo panel data based structural equations model, Research in Transport Economics, 25, 8-18.

Zahavi, Y. (1974) Travel Time Budgets and Mobility in Urban Areas, final report FHWA PL 8183, Federal Highway Administration, US Department of Transportation, Washington, D.C.

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