"In many older cities around the world, fire escape stairs may provide an alternate means of egress from upper floors. While not permitted for new construction, fire escape stairs may be a good solution for existing buildings where emergency escape is limited. Depending on the adopted building and fire codes in your jurisdiction, fire escape stairs should not provide more than 50 percent of the required means of egress. New fire escape stairs generally are permitted on existing buildings only where the code official has determined that alternative means of escape are impractical. Fire escape stairs should be located so a fire will not threaten persons leaving the building or affect the stairs' structural integrity."
Coffee Break Training - Fire Protection Series No. 2008-1
United States Fire Administration: http://www.usfa.fema.gov/