Example statement to determine linear chainage from line segment (PostgreSQL + PostGIS)

gistfile1.sql

/**
 * @lon = longitude of selected point
 * @lat = latitude of selected point
 */
SELECT 
   name,
   CAST(
      offset + 
      ST_Length(the_geom) * ST_Line_Locate_Point(the_geom, ST_Point(@lon, @lat))
   ) AS Double) 
   AS chainage 
FROM carriageway 
ORDER BY ST_Distance(the_geom, ST_Point(@lon, @lat)) ASC 
LIMIT 1