The requirement for emegrgancy lights (and exit signs) kick in once two exits are required from a space. This is dependant upon occupant load. Occupant load is established by the type of building or space (use and occupancy) and the square footage of the space.
From the 2000 IBC, here are the spaces with only one exit required:Assembly, business,educational, factory, mercantile and utility uses: 50 occupants. High hazard occupancies: 3 occupants (Very rare). Institutional (hospitals/jails and some assisted living)and residential (hotel/motel and some assisted living BUT NOT HOUSES): 10 occupants. Storage: 30 occupants.
Occupant load is additive as you travel towards the ultimate objective: the public way (street). Once two exits are require from a space, the corridor will need two exits, then the rest of the building and so on.
In a nut shell, if your building needs two exits (and most do) you need to provide emergancy lighting ALL THE WAY TO THE STREET!!!
I have heard that many areas don't enforce this. I'm not sure if a local ordinance trumps it or it is just not known or not enforced, but it is a HUGE cchange to those of us from ICBO land.
Salt Lake City