Right now, when making a component road, any lane to one side of a "centre-line" has traffic flow in one direction while those lanes on the other side of a "centreline" have traffic going in the opposite direction.
One problem is that when creating a multi-lane one way road, the "centre-line" is actually at the edge of the road.
Another problem occurs when a central Light Rail transit line, for example, is in the central section of a roadway, the lane directions are very difficult to inter-mix.
The idea is to allow the designer to choose the traffic flow direction of each lane independent of where the "centre-line" is drawn.