Understanding San Francisco's geography
Why San Francisco's drive times defy radius math.
The peninsula geography erases roughly half of any San Francisco isochrone — water consumes the western, northern, and eastern quadrants of most polygons. The four bridges (Bay, Golden Gate, San Mateo, Dumbarton) are the controlling variable for any cross-bay reach, and peak-hour congestion can swing reachable area by 50% or more. Steep topography in neighborhoods like Russian Hill, Nob Hill, Twin Peaks, and Bernal Heights also slows even short trips, and one-way street patterns in the city core add routing friction that default isochrone engines underestimate.
Retail teams should run drive-time and transit-time isochrones in parallel for SF — BART, Muni, and Caltrain meaningfully extend reach for residential catchments in ways driving alone misses. The post-2020 demographic shift in SF (population decline, downtown daytime population suppression, suburban Marin/East Bay/South Bay strength) means trade-area templates from before 2020 are stale; brands should rebuild SF isochrone-based studies from current data. Silicon Valley submarkets (Palo Alto, Mountain View, Cupertino, Sunnyvale) behave as nearly an independent high-income market.