Beyond Coupling-Coordination Scores: Digital-Transport Portfolio Balance and Urban Environmental Service Performance in China

Coupling-coordination scores are widely used to describe whether urban systems develop in step, although the usual index combines subsystem balance with average development level. This study separates these attributes for digital and transport infrastructure and relates them to urban environmental service performance in China. The analysis links city-level broadband use, mobile connectivity, data-center records, urban road provision, and seven municipal service indicators for 2002-2024. The preferred panel contains 6094 observations for 275 cities in 28 provinces. City fixed effects and province-by-year fixed effects absorb persistent local differences and common provincial shocks. After decomposing the conventional score, the standardized portfolio-level coefficient (0.188, p < 0.001) is more than four times the balance coefficient (0.045, p = 0.021). Balance remains positive before 2020 and when data-center capacity enters the digital index, but disappears with a road-density transport proxy. Component estimates locate the balance association in water and gas access, not sewage treatment, green coverage, or harmless waste treatment. The digital-transport interaction provides no evidence of superadditive returns. For SDG 9 and SDG 11 planning, infrastructure scale, relative balance, and external service outcomes should be monitored separately; synchronized construction alone does not establish environmental improvement.