If it's adequately controlled, yes. I've done several bridge demolitions where the only access to one side is to pass under the span being brought down, so you need to plan the work to get everyone and everything through in periods when it is safe to do so.

No, we phased the work, with safe periods in-between. So there's a period when people, plant and equipment has gone through, then a period of demolition, then a halt to demolition so stuff can be moved through. There needs to be a system with hold-points etc. so that not only is work stopped, but the structure remaining is known to be stable and sound and there's no loose debris anywhere it might teeter and fall off. With my structural designer hat on (rather than my H&S'er hat) I have designed crash decks. If they are crash decks you're actually dropping stuff on they become very significant structures very quickly. There's an incredible amount of energy in even quite moderate amounts of material falling from height. It's especially difficult if it might be a steel beam coming down end-on like a spear - a big lump of masonry (for example) tends to be less damaging at impact than an end-on steel beam. The only crash deck I've designed taht spans in its own right with people underneath was to be put up in Hong Kong. It used 600mm deep steel beams side-by-side (practically touching) with a thick steel plate across the top and a thick layer of gravel on top of that with a fabric mesh on top of that. They were dropping boulders from height though. The contractor took one look at the design and decided he'd move the people away rather than protect them in-place. That was what we had said he should do in the first place.

I have also done demolition projects with access routes underneath, it all depends on the type of demolition, could the project become more of a dismantle than a demolition, for example brick work can be taken down from scaffolding and chutes used to transport waste to the ground as opposed to using hammers to just knock the wall down. If you message me with more details I might be able to offer more advise from past experience.