Well the ones in the video actually had the road elevated rather than have a wooded overpass... it seems like that makes more sense longterm.

I think this is more a question of planting trees with roots that don't go too deep.

But, even if not - rebuilding them every 15 years might be cheaper than sound engineering for this problem space. After all, they don't have to handle 40 ton trucks or 50 cars at a time.

Agreed, but if you REALLY wanted to be sure, you could always put a large sheet of aluminum just above the bottom layer.

I thought there were problems with aluminum and plants.

From that viewpoint, the difference between this and a tunnel is minute.

They may be built the other way around, but you _can_ see these things as artificial hills with a tunnel dug through them. The effect of tree growth on their longevity will be similar.

That's going to happen no matter what. All you can really do is design them to be big and sturdy enough to last roughly as long as the road system. It doesn't make sense to try and severely outlive the roads that they cross.

It isn't that bad of a problem. Green roofs are becoming very common. It is just about the depth of the soil, the selection of the correct plants and usually some type of plastic barrier.