Peri-Urban Commercial Corridors as Post-Socialist Urban Frontiers: Spatial Gradients, Agglomeration Dynamics and Planning Implications in the Metropolitan Area of Constantine, Algeria

Spontaneous ribbon commercial development along peri-urban national roads represents a defining feature of post-socialist metropolitan expansion in the Maghreb, yet it has received no systematic quantitative analysis at metropolitan scale. Drawing on an exhaustive GPS-referenced field survey of 927 commercial units along five radial corridors in Constantine, Algeria’s third largest city, this study develops three original spatial indices — Commercial Density Index (CDI), Functional Specialisation Index (FSI), and Correspondence Score Index (CSI) — and applies a four-phase Sequential Mixed-method Commercial-corridor Analysis (SMCA) protocol to thirteen commercial sequences. Findings reveal a statistically robust urban–rural specialisation gradient (Spearman ρ = −0.94, p = 0.017 along the RN27 corridor), self-reinforcing Marshallian agglomeration dynamics in high-FSI clusters (Moran’s I > 0.70, p < 0.001), and two systematic deviations from Berry’s (1963) canonical typology: the structural embedding of wholesale logistics within retail units — a product of post-1989 import liberalisation — and a resource-triggered genesis pattern observed at Chaaba Rssas. These context-specific deviations justify a new variant, the Highway-Oriented Enterprise-Centred (HOEC) model, and the introduction of a Peripheral Commercial Activity Zone (PCAZ) in Algerian planning law. The SMCA protocol is proposed as a reproducible framework for comparable data-scarce environments across the MENA region.