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.
Here in my neck of the woods( just north of Houston) I personally have never installed an exterior light on the emergency system. Nor have I seen any drawings calling out for the exterior lights to be on an emergency circuit.
A large strip mall in this area, about 3 years old has exit signs outside!!
The grade level facing the highway has an "overhang" that is 30' deep; has a sidewalk, and is the "front" for the stores. The upper level of this overhang is the loading area for the upper level stores. At grade level, there are "EXIT" signs outside.
In other words, you exit the stores individually, are outside, and there are "exit" signs mounted on the underside of the overhang. BTW, ALL of the exit/emergency units within this center are on back-up generators.