with poly as  (
    SELECT ST_UNION(ST_BUFFER(lines.geom, 3)) as newgeom
        FROM roads
)
select
    case
    when ST_coveredby(l.geom, poly.newgeom)
    then l.geom
    else
        ST_Intersection(geom, poly.newgeom) end as geom
from lines as l
    inner join poly
        on ST_Intersects(l.geom, poly.newgeom)
            and not ST_touches(l.geom, poly.newgeom)