Creating polygons of area surrounded by roads / lines using PostGIS?

Some hints:

1. You could use the ST_Polygonize processing of PostGIS.
2. You could have a look at this QGIS plugin (I have not tested it myself...)
3. If you speak Java, you could use the polygoniser of JTS.

You could try creating a buffer layer of your roads, so that it is a polygon. Then create a polygon layer delineating your area of interest, and use the union tool on both polygon layers. You should be able to differentiate the polygons inside the roads (white areas) from the transportation lines from the union results.