Marchetti’s Constant is a term for the average amount of time spent travelling each day, which is about one and a half hours.*

Developed by Venetian physicist Cesare Marchetti, it posits that although forms of urban planning and transport may change, and although some live in villages and others in cities, people gradually adjust their lives to their conditions (including the location of their homes in relation to their workplace) such that the average travel time stays approximately constant.

Even since Neolithic times, people have kept the time at which they travel per day the same, even though the distance may increase. Once people have to travel longer than an hour and a half a day, they do something to get to work faster—changing where they live, where they work, or how they get from place to place.

The prominent Australian environmental scientist Peter Newman often makes reference to Marchetti’s Constant in his arguments for sustainable urban planning and transport:

[T]he city is always one-hour wide. The walking cities of the past – historic, medieval cities – were five to eight kilometres wide. You could walk across them in an hour. Victorian cities, the industrial revolution cities, spread out because the pipes and the rails meant that we could now travel 20 to 30 kilometres. And the city remained one hour wide.

But the new frontier entered essentially by US traffic engineers was to spread the city out further around highways. So the city spread out and in an hour you could go 50 kilometres.

The Marchetti principle does mean that if you have a good public transport system there will be a market for dense, walkable development.