Beyond the 3-30-300 Rule: Construction of a Scalable Composite Index for the Evaluation of Urban Green—The Ferrara Case Study

The 3-30-300 rule, proposed by Cecil Konijnendijk, is oriented towards the design of greener cities. However, subsequent literature has revealed some application limits due to overly simple definitions (visibility of 3 trees), fixed thresholds (30% tree cover) and theoretical distances (300 m to the park) that do not consider ecological quality, real green area proximity and possible socio-demographic differences. The present research attempts to overcome these limitations through the elaboration of a scalable composite index that, starting from the original rule, integrates ecological, infrastructural and population variables to give a more robust measure of the availability and usability of urban green. The index was tested in the study area of the urban centre of Ferrara (Italy). Three sub-indices were calculated for each building: Indicator 3—Visibility (I3), Indicator 30—Tree cover (I30), and Indicator 300—Green area proximity (I300). Once normalized and weighted, the three indicators were aggregated into a composite index conceived as a scalable and replicable framework adaptable to diverse urban settings. By spatially integrating population data, the methodology explicitly embeds the distributional dimension of climate justice, supporting evidence-based adaptation strategies and equitable urban regeneration policies. Moving beyond the binary logic of the original 3-30-300 rule, the approach provides an operational decision-support tool to detect intra-urban inequalities, to address just green transitions and to monitor urban greening interventions over time.