For roads. I once had two roads meet while they were going downward from two opposite hills. I had two options, make two ramps to the lower layer of the hill or merge them and form a diagonal road to the lower layer. The merged diagonal road saved me more space. I did have another option of merging them going to on of the two directions that they're already leading, but that didn't turn out to look very appealing. Hope you can understand what I'm saying.