From the sounds of it, expedited freight is much more intertwined with "normal" freight.
It seems like everything in the transportation industry is becoming intertwined and even trying to pigeon hole a specific service like expedite with a narrow definition is nearly impossible. But, my definition of expedited freight is freight that requires: Nonstop, dock to dock, exclusive use service with on demand, 24/7 pick-up and delivery times. This definition includes ground and air freight service and a combination of both.
Most large LTL companies offer some form of expedited freight service. These services may include "expedite" in the name or priority, premium, platinum or some other adjective, but don't meet my definition of expedite freight. When an LTL carrier is about to incur a service failure on it's higher priced "expedite" freight they call in a true expedite carrier to save their butt.
Once you can come up with a working definition of expedite freight to fit your particular situation you can start looking for statistics. You mentioned in a prior post that your boss would be happy. If you are with some type of transportation company your company may subscribe to various transportation rags.
Traffic World has done numerous articles on expediting, many by John D. Schulz. A
Traffic World subscription will get you access to their on-line library. Another source for expedite statistics is Satish Jindel of SJ Consulting Group. Do some on-line research starting with these two leads. Good luck and please post a condensed version of your findings here on E.O.