Urban air mobility (UAM) using passenger drones has drawn considerable attention as a next-generation mobility solution. UAM may be an alternative for realizing carbon neutrality and for solving traffic congestion in large cities. We designed a three-dimensional movement route model of a passenger drone and examined the environmental impact to inform Seoul's urban air traffic (UAM) policy. This study explored the safety route of large drones using 3D spatial information elements based on a weighted entropy function. Life cycle assessment was used to evaluate the environmental impact of replacing internal combustion engine vehicles (ICEV) with passenger drones. We found that safe routes include high-frequency routes traveling along a stream or river to minimize human and material damage even if the drone would fall when moving along a stream or river. The analysis of the amount of pollutant emissions based on the traffic volume of Seoul shows that current emissions are 4.7 times the estimated emissions from passenger drones. Since environmental damage can be controlled based on the national energy supply and demand policy, further in-depth research should be conducted.