Synergistic effects of low-carbon city pilot and the ‘Broadband China’ pilot policy: an evaluation based on urban green economic efficiency

Against the strategic background of synergistic digital and green transformation, this study takes the ‘Broadband China’ strategy and low-carbon city pilot policies as a quasi-natural experiment. Using panel data from 278 prefecture-level cities from 2011 to 2022, we employ a multi-period difference-in-differences model to examine how the coordinated implementation of these two policies affects urban green economic efficiency. Results show that the dual-pilot policy enhanced urban green economic efficiency by approximately 2.76%, roughly 1.6 times greater than the single-pilot policy and indicating a clear synergistic outcome (‘1 + 1 > 2’). This result remains robust across a series of sensitivity analyses. Mechanism analysis indicates that the policies promote digital industry and population agglomeration while restraining financial agglomeration. The positive impacts are strongest in eastern, larger and less-polluted cities, and sequencing the policies as ‘low-carbon first, broadband second’ yields the greatest synergistic effect. In addition, dual pilot policies generate positive spatial spillovers to neighbouring cities. These findings provide new empirical evidence on the micro-mechanisms of digital-green integration and offer practical policy guidance for optimizing strategy portfolios and advancing regionally coordinated green transformation.