Feasibility of horizontal evacuation after flooding onset and its relationship with geographical characteristics: Insights from past flood events in Japan

The significance of early evacuation during floods is well-established. However, few studies have assessed evacuation behavior after inundation such as route selection and associated geographical constraints empirically. This study aims to clarify the feasibility of horizontal evacuation after inundation and its association with geographical characteristics. We conducted multi-agent evacuation simulations across five districts impacted by three flood cases in Japan, followed by clustering and statistical analyses, to determine the influence of geographical factors. Results revealed that evacuation success rates were high when evacuation was initiated before inundation but declined sharply thereafter. Of the three evaluated routes—evacuating via the shortest possible route (SHORTEST), evacuating under exclusion of impassable roads via the shortest possible route (EEIR-SPR), and evacuating inundated areas via the shortest possible route (EIA-SPR)—EEIR-SPR and EIA-SPR improved success rates compared with SHORTEST, with a maximum increase of 36 %. However, the additional benefit of EIA-SPR over EEIR-SPR was modest, and some evacuees succeeded only with EEIR-SPR. While avoiding inundated areas is critical, not all evacuees benefit from following the shortest non-flooded route. Geographical characteristics influenced evacuation feasibility: residents at higher elevations and closer to shelters had higher success rates across all routes, whereas those in low-lying areas with limited road networks faced significant evacuation challenges. Thus, it is important to incorporate geographical risk assessments into evacuation planning and customize strategies to local terrain and infrastructure conditions. This study provides fundamental insights for enhancing evacuation planning and establishing standardized evaluation criteria for evacuation strategies.