Per Commercial, Industrial, Multi-Family Dwellings and Hospital Projects, the City of LA has required the "Low" Exit Signs, in addition to the standard over-the-egress-door Signage, since apx. 1993.
LADBS allows Self-Illuminated (Tritium) Signage for the lower Exit Signs.
IIRC, the lower Signs are mounted +24" AFF to Center.
BTW; out here (So. Ca.), the Trade Slang term for Self-Illuminated Exit Signage is "Nuclear Exit Signs"
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!