A human operator would see a moving crane and say "that's a construction site, I'm going to go around". They would not fly directly under a crane even if it visually looked clear. In this case the crane was actively lowering something, so the drone not only missed the cable but it flew directly in between the crane and a visible object hanging in the air below the crane.
For concrete numbers, I would say stay 50 yards away from construction equipment, and always laterally or above, not below. Honestly these drones are enormous so I think "don't go under" can just be a blanket rule. They can't be going under trees or bridges or overpasses either, they're too big.
Edit: Also, the drones themselves should be far enough apart that if one crashes the other has time to react and stop or change course. I don't have a concrete number there, it depends on their speed and acceleration, but they shouldn't be flying so close that if one crashes they all will.
For concrete numbers, I would say stay 50 yards away from construction equipment, and always laterally or above, not below. Honestly these drones are enormous so I think "don't go under" can just be a blanket rule. They can't be going under trees or bridges or overpasses either, they're too big.
Edit: Also, the drones themselves should be far enough apart that if one crashes the other has time to react and stop or change course. I don't have a concrete number there, it depends on their speed and acceleration, but they shouldn't be flying so close that if one crashes they all will.