Perhaps most basic, though, is that in terms of urban living the car has become a victim of its own success. In 1994 the physicist Cesare Marchetti argued that people budget an average travel time of around one hour getting to work; they are unwilling to spend more. For decades cars allowed this budget to go farther. But as suburbs grow and congestion increases most cities eventually hit a “sprawl wall” of too-long commutes beyond which they will not spread far. After that, it appears, a significant number of people start to move back towards the city centre. In America, where over 50% of the population lives in suburbs, more than half the nation’s 51 largest cities are seeing more growth in the core than outside it, according to William Frey at the Brookings Institution.