If you have data covering a large area (across multiple projected coordinate systems), it could be helpful to use the PostGIS *geography* type instead of *geometry*.
Especially if your base data ist already in WGS84 GCS (SRID 4326).

The ST_Distance function uses geodetic calculation and returns distance in meters.

Some additional explanation can be found here:
[Pros and Cons of geography / geometry types][1]
and 
[Difference between geography / geometry types][2]


  [1]: https://gis.stackexchange.com/questions/6681/what-are-the-pros-and-cons-of-postgis-geography-and-geometry-types
  [2]: https://gis.stackexchange.com/questions/26082/what-is-the-difference-between-geometric-and-geographic-columns