I have a shapefile from OSM which contains all the roads within a city. Larger roads (like dual-carriageways) seem to have 2 parallel lines. I'd like to collapse / combine those into 1 line so I can export and render them nicely in an external 3D application (2 overlapping roads will be complicated and will look weird in 3D).
How can I achieve this with either QGIS or PostGIS? I am not worried about a slight loss of accuracy (within a few metres) and would like the resulting, single line to be between (ideally the centre of) the current, parallel lines.
(here is an example of the dual road lines that I want to combine)