How much cost/effort would it be to string up safety nets in urban areas for drones? My biggest concern with drones is that they fail, and that means they're l

How much cost/effort would it be to string up safety nets in urban areas for drones? My biggest concern with drones is that they fail, and that means they're landing on someone's head. If a city invested in nets, maybe 100ft above the ground, I imagine you could have a much less anxious civilian population with drones flying overhead. Someone tell me why this is a silly idea.