Employment-access constraints in the 15-minute city

The “15-minute city” has emerged as an influential urban-planning paradigm, promoting proximity to work and essential services. Its feasibility for journey-to-work commuting, however, has rarely been examined quantitatively. Here, we show that employment proximity is constrained by heterogeneity in the number of employees per firm. Combining urban geometry with empirically observed firm-size distributions, we derive a lower bound on commuting time that holds even under optimal workplace placement. This bound reveals a sharp transition: when employment is sufficiently unevenly distributed across firms, no spatial rearrangement of workplaces can provide uniformly short commutes. Applied to Paris and its near suburbs, with area A = 762 km 2 and empirical firm-size exponent γ ^ ≃ 1.38 , the model predicts that only about 37% of workers can reach their workplace within 15 minutes by walking. Cycling increases this share to about 67%, but still leaves roughly one third of the workforce beyond the 15-minute threshold. Achieving universal short commutes would therefore require economic restructuring, differentiated mobility strategies, remote work, or a longer target time. Our results suggest replacing the universal 15-minute target, for commuting, by a city-specific benchmark determined by firm-size heterogeneity and urban scale.