Hey txesmi, thanks for your answer but there are several problems with your approach.
How do I find the neighbour? My roads consist of ~100 vertices.
What if a shape is not closed? If you watch a voronoi diagramm it has ends that don't form a polygon.
Furthermore, there can be more than 2 networks that are not connected. Again a special case that is not covered by your idea.
But thanks anyway! I'm very glad you are thinking with me!