offsetting the lines is not perfect as we can't define how the join in curves is done. see http://postgis.refractions.net/docs/ST_OffsetCurve.html for an alternative.
- Either use this postgis function to offset line
or
- Hack Mapnik so that an offset-join parameter is added to LineSymbolizer and LinePatternSymbolizer.
offsetting the lines is not perfect as we can't define how the join in curves is done. see http://postgis.refractions.net/docs/ST_OffsetCurve.html for an alternative.
or