This code creates another st_line_interpolate_point with a 3rd argument for offset of the point to the left or right side of the line segment. Useful for Geocoding.

Example Usage:
{{{
--Example: Geocode on a forward range from TIGER line file (Assuming integer range fields)
select 
st_line_interpolate_point(te.the_geom, 
	((12345 - fromhn::float) / (tohn::float - fromhn::float)),
	case when side = 'R' then .0001 else -.0001 end
),
'A house'
from tl_edges te
join tl_addr ta on te.tlid = ta.tlid
where fullname = 'Main St' and 12345 between fromhn::int and tohn::int and zip = '92333'
and (12345 % 2) = (fromhn::int % 2)
and fromhn::int < tohn::int;
}}}

Function Definition:
{{{
create or replace function st_line_interpolate_point(m_geom geometry, m_percent double precision, m_offset double precision) 
returns geometry as
$BODY$

/*
	Interpolate a point across a line and then offset to the left ( negative ) or right by m_offset distance
	m_percent 0 to 1

*/

declare m_seg geometry;
declare p1 geometry;
declare p2 geometry;

BEGIN

m_seg :=  st_linesubstring(m_geom,0, m_percent);

/* Grab the 2 points of the matching line segment */
p1:=st_pointn(m_seg,st_npoints(m_seg)-1);
p2:=st_endpoint(m_seg);

/* get the delta of the line segment and offset the interpolated point */
/* Rotate deltas by 90 degrees (invert dx/dy ) */
/* spheroid projections will need the offset to vary based on lat/lon */
return st_translate(
	p2,
	((st_y(p2) - st_y(p1)) / st_distance(p1,p2)) * m_offset,
	((st_x(p2) - st_x(p1)) / st_distance(p1,p2)) * (-m_offset)
);

END;

$BODY$
language plpgsql;
}}}