I'm not at all sure that a ducted propellor would increase efficiency as drag losses in and around the duct would be much larger than the relatively small improvements at the blade tips. Not to mention the extra weight of the duct and supporting structure.
If they were more efficient we'd see them, as it is we see virtually none and for good reasons.
I'm not sure that bystander safety is relevant. What would bystanders be doing close to running aircraft?
Ducted props are pretty pointless in 99.9% of applications.