It's hard not to be enthusiastic about the 15-minute city, la ville du quart d’heure in the Paris rollout. It would reduce the need for a car and change so much in the life of the individual city-dweller, the volume of traffic and emissions.

Way back, in 1994, Italian physicist Cesare Marchetti identified a nearly universal rule of human behaviour: People tend to travel for about one hour per day, in total. It became known as Marchetti’s Constant and explains the ways ancient and newer cities alike have developed through time. So to the numbers:

3 miles or 5 km: That’s the average diameter of Europe’s oldest cities, including Rome, Venice, and Berlin, before 1800, when people walked to work.

1 to 2 miles: Distance most people can walk in 30 minutes