What are your choices if demand is 24 hours? If you have a large fleet you staff whenever there is demand. Since it is easier to get parts and outside services during the day the day shift might be heaviest. This is tough situation for a small fleet with 24 hour demand. In this case you might resort to an outside service, 24 hour garage, or a call-out list or make a deal with another local fleet needing coverage and share the responsibility.