During heavy rainfall disasters, local municipalities face difficulties in decision making regarding evacuation, though they are required to provide the residents with accurate information. Due to its wide range of interpretation, the guidelines for evacuation by the Japanese Government cannot always be applicable for actual cases across Japan. Each municipality relies on local administrators’ empirical knowledge, resulting in non-objective decisions to evacuate. The vulnerability, consisting of calculating disaster risk and hazard, stems from administrators’ experiences. We present an innovative way to assess accurate evacuation times for disaster preparedness and present time limits in a form that can be used by local government officials for decision making.