Simplifying polygons to linestring?

What you need is called "skeletisation algorithm".

Have a look at this article:

Haunert, J.-H., Sester, M., Jun. 2008. Area collapse and road centerlines based on straight skeletons. GeoInformatica 12 (2), 169-191. URL http://dx.doi.org/10.1007/s10707-007-0028-x

Some of these algorithms are developped in the CGAL library.

In PostGIS 2.2 with SFCGAL, this can be done with ST_StraightSkeleton or ST_ApproximateMedialAxis, depending on your criteria.

the JUMP skeletonizer plugin is available as part of openjump http://www.openjump.org