Cutting a segment
You can cut a segment into two new segments. You can do this by adding a new segment from some random point nearby, to the point of the segment where you want to cut it. A new junction will be added. Delete the new segment just created. The newly created junction will remain and you made two new segments out of one. If you need to actually separate the road segments at that point, just relocate the end point of one of the two new segments.