1. These are Lines, they guide everything :

2. These are paths, it's simply a texture that follow a curve :

3. Lanes are path with a line on the left and right side :

4. Between the lines, there is a surface (asphalt by default) :

5. At the intersection of two lines, the lines they are splitted...

6. So when you intersect two Lanes, you can configure each part at each intersection :

7. When you have two close Lanes, it may have indesirable intersections :

8. For Roundabout with multiple Lanes, simply create two Roundabout at the same position but with different radius, and disable the right Line of the bigger one (or the left of the smaller) :

Have fun !