### Comment:
I liked black science man's take on it. One lane is static. All traffic at same speed or worse. Two lanes already lets you move in two dimensions and pass slow traffic, so you've gained that ability. A third dimension (height) doesn't actually add any new capability, it just requires more fuel and makes things more complicated. It's easier to just add another lane.

### REPLY:
a caveat is adding lanes has diminishing returns.

### END.