>The earlier ones hit a crane which one could argue was an edge case as a temporary structure.
I would expect them not to fly into any kind of structure. That they'd hit a crane is pretty insane considering what the results of something like that could be.
I would expect the result to be the same as running into anything else: drone and any payload crash into the ground.
Drones are lightweight, they're not going to do much to heavy machinery. Basically the same as a brick wall.
The real fear is propellers hitting a human. The result is not good at all.
Zero? I think the expected number of collisions can be larger than zero. Jimmy Johns sandwich delivery by bicycle has resulted in more collisions than zero and that is arguably safe.
You are setting an impossible standard.