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 and Difference between geography / geometry types
