What's the point of required upgrade from 'Six-lane Road' to 'Avenue'?

As with other granular elements in this game, it is perhaps best not to spend too much time pondering the logic of this point. It is what it is.

If it helps, perhaps your citizens are happier looking at the palm trees on the avenues while they are stuck in traffic jams? They will strangely be even happier looking at streetcars in 4 lane roads.