On average, each person spends around 70 minutes per day on the move. This phenomenon, known as the "travel time budget" (TTB), has been extensively studied and found to be valid regardless of historical era, city, culture or means of transport.
In other words, all cities could be considered as being 70 minutes wide, and this has large implications for urban development. For example, cities structured around walking have a much smaller physical radius than cities structured around private or public transport.
In this article we will examine some of the findings and conclusions from the Urban Insight report "Running to stand still – the role of travel time in transport planning".
How travel defines a city's characteristics
While a city's area coverage, density and population all change over time, its citizens' travel time remains relatively constant. Research has suggested that humans are biologically programmed not to travel much more than an hour a day, and that this routine stretches back in time much longer than a century.
Economists might define this as a question of utility maximisation. The travel time budget would then express the time a person is willing to spend on travel in order to gain access to the supply of activities she or he desires. At a certain threshold, a point is reached where the increase in travel time does not pay off in terms of activities.
Another explanation points to the fact that travel is one of many activities that takes up time in people’s daily lives. A substantial amount of time is devoted to work, leisure, household chores and sleep, making it difficult or inconvenient for most people to allocate much more than 90 minutes for travel.
As a result, many cities have historically grown at the same rate as that enabled by increases in travel speed. An illustrative example is the way motorisation of European cities
during the mid-20th century (almost naturally) coincided with the dominance of Modernist urban development.
In recent years, the travel time budget appears to have increased slightly. This is not necessarily due to faster means of transport, but more likely because of increased travel comfort – including the possibility to combine travel with computer or phone use.
Regardless, travel stands out as one of the predominant behaviours affecting the size, characteristics and function of cities.