Laying down roads is just like laying down track, as far as junctions are concerned.
For intersections when I decide I want to place a 3- or 4-way interscetion in the middle of a previously laid stretch of road I just insert two spline points in the approximate location where I want the intersection, delete the piece of road between them, place the intersection, then drag the road ends to the connector points on the intersection.
Hope that makes sense.
