Marchetti’s Constant and Travel Time Budgets: One hour’s travel a day?
The location of villages in Greece, the average daily commute, and the size of cities – what do these seemingly unrelated things have in common? According to the Italian systems analyst Cesare Marchetti (who turned 90 last year), they can all be related to the finding that humans spend on average one hour per day travelling. This notion that each of us on average has one hour of daily travel time – or a one hour ‘travel time budget’ – is often known as ‘Marchetti’s constant’.
In the 1990s, Marchetti used this concept in an influential academic paper titled Anthropological Invariants in Travel Behaviour, in which he argued that in all cultures across the globe and throughout history humans allocate on average one hour per day to travelling and that this “quintessential unity of travelling instincts” can explain the way humanity has formed its settlements. According to Marchetti, Greek villages occur in such a way that they have a hinterland of around 20km2, so that the journey from a village to the edge of its territory is about an hour’s round trip at walking pace. Likewise, Marchetti argues that the size of cities increases in proportion to the speed of their transportation systems, so that the periphery is not much more than 30 minutes’ journey (i.e. a one hour round trip) from its core. In this view, it is the speed of transportation systems that sets a limit on city size. “Hypersonic airplanes promise to glue the world into a single territory, the famous global village,” predicts Marchetti.
These are bold claims, with profound implications for our understanding of human travel behaviour and for transport and urban planning. But does the basic idea stand up to scrutiny: is there a universal ‘travel time budget’ of one hour that is constant across societies?
Although the one hour figure is often known as Marchetti’s constant, Marchetti himself attributed the finding to transportation engineer Yacov Zahavi. In his studies of transport modeling in the 1970s, Zahavi came up with the concepts of the travel time budget (TTB) and the travel money budget (TMB). Zahavi argued that travellers tend to combine these budgets in order to maximize the distance they can travel within their constraints of time and money. Despite Marchetti’s interpretation of Zahavi’s work, it seems that Zahavi himself was not convinced of a universal human TTB, rather his work indicated that a TTB is “stable and predictable, but not necessarily constant. It can, for example, change over time, vary from one place to another, and needs to be established by measurement before being used.” Zahavi found that socioeconomic status and the speed of a city’s transportation system both had a strong impact on the TTB of travellers in a given location. He found that poorer residents in developing countries, where average speeds were slow, devoted an average of an hour and a half to daily travel, and two hours in some cities: well in excess of Marchetti’s constant of only one hour.
Since the 1970s, there have been numerous studies examining the idea of a constant TTB. In their survey of this literature, Mokhtarian and Chen found that the evidence shows that the amount of time people spend on daily travel is not a universal constant, “except, perhaps, at the most aggregate level.”  Findings from a more recent study by Kung et al., utilizing large data sets gathered from mobile phone call records, are “consistent with Mokhtarian and Chen’s claims that the constant travel time hypothesis applies only at the aggregate level” but the authors felt they could not conclusively argue against Marchetti’s constant .
The idea of a constant one hour TTB might not be as straightforward as Marchetti described – but it might still be a useful concept. Even if the one hour figure is only a mean average, it is one around which a large proportion of people in a wide range of societies do cluster. Also, it does seem that when the speed of a transport mode increases people tend to use this faster option to travel further, rather than to reduce their travel time. We still don’t fully understand the reasons behind this but it may indicate a basic human appetite for travel. And, as with most of our appetites, there may be an optimum amount of travel that fulfills but does not exceed that desire. This may vary between individuals to some degree but tend towards an hour daily. This interpretation is supported by research into the well being of commuters, which has found that when people have a journey to work of over 30 minutes (i.e. one hour return travel daily), each additional minute of travel negatively impacts the person’s feelings of their daily activities being worthwhile; and for journeys over 45 minutes, life satisfaction also becomes negatively affected. 
If, as this research suggests, there is indeed a basic human appetite for travel, this has profound implications for how we plan our lives: our cities, transport systems, working and leisure practices. Understanding the basic drivers behind our desire to travel is therefore crucial for planning for the future. With this in mind the Independent Transport Commission has created the Why Travel? project, which aims to better understand why people travel and thus to improve decision making about travel at an individual and policy level. The project has engaged experts from across the arts and sciences to explore these issues and to offer important lessons for the future of travel. For more information on the project, including news stories and videos, see www.whytravel.org
 Marchetti, C., 1994: Anthropological Invariants in Travel Behavior, Technological Forecasting and Social Change , 47 :75–88, Internal Publication, International Institute for Applied Systems Analysis, Laxenburg, Austria
 http://www.surveyarchive.org/zahavi.html and Zahavi, Yacov (1979) “UMOT” Project. Prepared for U.S. Department of Transportation, Washington, D. C. and Ministry of Transport, Federal Republic of Germany, Bonn. Rept. DOTRSPA-DPB-20-79-3, August.
 Mokhtarian, P. L. and Chen, C., 2004: TTB or not TTB, that is the question: a review and analysis of the empirical literature on travel time (and money) budgets. Transportation Research Part A-Policy and Practice, 38(9-10) https://escholarship.org/uc/item/3kr185ts
 Kevin S. Kung , Kael Greco, Stanislav Sobolevsky, Carlo Ratti, 2014: Exploring Universal Patterns in Human Home-Work Commuting from Mobile Phone Data. PLOS One doi.org/10.1371/journal.pone.0096180
 Mokhtarian and Salomon proposed that we might usefully think about a travel time budget as distinct from travel time expenditure i.e. that what we are measuring is the amount of time people actually spend on travel but that there may also be “an unobserved ideal travel time budget” and that people may try to adjust their travel behaviours towards that ideal time: Mokhtarian, P. L. and Salomon, I. (2001) How Derived is the Demand for Travel? Some Conceptual and Measurement Considerations. Transportation Research A 35(8) , 695-719.