Finding a safe evacuation route is important in the event of an accident or natural disaster. Along with climate change in recent years, torrential rains have become more occasional than before in both urban and rural areas. Intensive and continuing rainfall may lead to disasters such as landslides and inundations. Even if a single point on an evacuation route is inundated or critically damaged, then the route is unavailable and the evacuees who were planning to use it must find other routes. In addition, in lowland districts in urban areas, where a large number of local people reside, might rush on a limited number of available roads, particularly when a mandatory evacuation is alerted. This may raise another type of risk by excessive human flow or traffic congestion. Most hazard maps are created based on geographical elevations, which quantifies the extent of flood risk, typically in terms of inundation depth. Those kinds of maps may be useful for averting or mitigating property damage, while it is not tied with safeness of evacuation routes. In the case of an inundation in lowland areas, a large number of roads in the vicinity of rivers would become inundated and unavailable. This type of temporal risk due to congestion has not been considered to date. This study along this line considers both geographical elevations and availability of roads. A lowland district in eastern Tokyo, Adachi City, is focused on for assessment. Empirical findings highlight the impact of traffic congestion and isolation of flooded areas.