Walkable Access to Earthquake Assembly Points Considering Building Mass–Street Geometry (H/W) Risks: A Comparative Neighbourhood-Scale Analysis in Istanbul

Wide streets are often assumed to keep post-earthquake walking corridors open. In four Istanbul neighbourhoods, a morphological height-to-canyon-width (H/W) screening produces the opposite ranking: 300 m earthquake-assembly-point (EAP) coverage contracted by 72.2% in the planned/grid pair (Erenköy and Göztepe) and by 30.1% in the dense/organic pair (Merkezefendi and Çırpıcı). Median canyon width is 4.8 m larger in the Kadıköy pair, yet median flanking height is 11.7 m larger, so median H/W is higher. OpenStreetMap (OSM) geometry from August 2026 was coupled with Istanbul Metropolitan Municipality (IMM) 2017 neighbourhood tables and 5532 assembly polygons. Heights used OSM tags for 49.0% of 3993 footprints and IMM floor-class means otherwise. Scenario A used the full walking graph; Scenario B removed segments with H/W ≥ 1.0 as a screening rule, not as predicted collapse. Walking distance includes building-to-street and EAP-to-street snaps. The ranking held at H/W = 0.8 and 1.2 and under ±20% imputed-height scaling. Hypothetical access nodes raised coverage to 72.3% (dense) and 44.0% (planned), but 18 of 81 centroids fall on OSM footprints, marking gaps rather than parcels. Unobstructed-network maps overstate post-earthquake walkability; H/W screening is a design-support layer, not a damage forecast.