The The Vertical-Horizontal Paradox: PLUS-Based Simulation of Four Decades of Urban Intensification and Three Future Scenarios in Hawassa, Ethiopia

Rapid urbanization across sub‑Saharan Africa challenges statutory planning instruments. Using a PLUS model, we examine the “Vertical‑Horizontal Paradox” the decoupling of designated vertical intensification zones from observed urban expansion in Hawassa, Ethiopia (1984-2024). Population density dominated early growth (0.328-0.343); after 2004, precipitation (0.129-0.183) and accessibility (0.118) gained prominence. The building height proxy vertical intensification potential from the 2019 Structure Plan exhibited negligible statistical association with 2014-2024 expansion. Model validation was robust (accuracy 82.5-85.2%; FoM 0.159-0.369). Projections to 2054 reveal stark trade‑offs: Ecological Protection curtails urban area by 25% (-1,190 ha) relative to Business‑as‑Usual. Urban expansion aligns more strongly with precipitation, accessibility, and demographic momentum than with zoning a “paper intensification” underscoring the need for planning instruments grounded in actual drivers of change. This study offers a reproducible template for testing planning efficacy and introduces regulatory zoning as a quantifiable driver within machine learning‑cellular automata frameworks. Ethiopian Journal of Science and Sustainable Development, Vol 13 No 2 (2026)