Marchetti’s idea is that people have a daily travel budget of around an hour that they choose to spend in different ways, picking transport options that fill up that time. If we live close to work, we might walk or cycle. If roads or public transport improve, we might move further away. But whether we drive, cycle, walk or take public transport we will spend roughly the same amount of time doing it.

Of course, the number is an average and there are outliers. But since putting forward the idea in 1994, using data that US transportation engineer Yacov Zahavi collected in the 50s and 60s, it has been backed up by several other studies. It also applies over time. A 2013 study comparing average commuting times in the US in 1980 and 2010 found that they had changed little in 30 years, despite enormous improvements to public transport.

It also seems to apply around the world. In 2014 a team from the Senseable City Lab at the Massachusetts Institute of Technology used mobile phone data to compare commuting times in different countries. Again they found that times remained relatively constant despite people travelling different distances.

For Marchetti, one hour is the basic limit for the total amount of travel that humans have been willing to put up with each day since the dawn of human society. He speculated that early humans travelling on foot at around 5km/h (3mph) would thus have a territory radius of 2.5km. To test the idea, he looked at the areas associated with individual villages in Greece – territories established over many centuries – and found that they tended to be roughly 5km across, supporting his claim.