Policy coordination as a diffusion accelerator: Spatial spillovers in electric mobility adoption

Electric mobility transitions in emerging economies are constrained not only by price and infrastructure but by the coherence of governance frameworks through which public interventions are delivered. Yet the role of policy coordination as a diffusion mechanism-activating spatial network externalities rather than merely supplementing instrument design-remains theoretically underdeveloped and empirically unexplored at sub-national scale. This paper addresses that gap. Using a province-level panel of 63 Vietnamese administrative units over 2010-2023 (882 province-year observations), we construct a seven-dimension Policy Coordination Index (PCI) from Ministry of Transport monitoring reports, Provincial Competitiveness Index enterprise survey data, and EVN electricity operator records, and estimate Spatial Durbin Models (SDM) to identify direct and cross-provincial spillover effects on electric vehicle (EV) adoption rates. Endogeneity is addressed through a two-instrument strategy: historical administrative capacity (2005) and pre-internet telecommunications density (1999-2000), yielding first-stage F = 58.4 and a passing overidentification test (Hansen J = 0.210). Results indicate that a one-standard-deviation increase in policy coordination raises provincial EV adoption by 1.84 percentage points directly; spatial spillovers contribute an additional 38-43% of total effects (indirect effect: 1.18 pp; total: 3.02 pp). Panel threshold analysis (Hansen, 1999) identifies a critical institutional quality threshold γ⁎ = 0.47, below which coordination effects are attenuated by a factor of 2.9. Mediation analysis confirms that charging infrastructure density accounts for 31% of the total coordination effect, with the dominant 69% operating through direct governance predictability channels. These findings reframe policy coordination as a general-purpose governance technology that enables-not merely accompanies-transport decarbonisation, with direct implications for sequenced multi-level governance reform in emerging economies.