  Commuting is a key mechanism that governs the dynamics of cities. Despite its
importance, very little is known of the properties and mechanisms underlying
this crucial urban process. Here, we capitalize on $\sim$ 50 million
individuals' smartphone data from 234 Chinese cities to show that urban
commuting obeys remarkable regularities. These regularities can be generalized
as two laws: (i) the scale-invariance of the average commuting distance across
cities, which is a long-awaited validation of Marchetti's constant conjecture,
and (ii) a universal inverted U-shape of the commuting distance as a function
of the distance from the city centre within cities, indicating that the city
centre's attraction is bounded. Motivated by such empirical findings, we
develop a simple urban growth model that connects individual-level mobility
choices with macroscopic urban spatial structure and faithfully explains both
commuting laws. Our results further show that the scale-invariants of human
mobility will ultimately lead to the polycentric transition in cities, which
could be used to better inform urban development strategies.
