speed loss dynamic

Hi, I am trying to assemble some information regarding average speeds. Anyone who has driven a truck for an appreciable legnth of time knows that the speed you average will be slower than the posted speed limit. Furthermore, your average speed will be less than whatever your truck is governed at. For lack of a better term, and ignorance of whatever may be published about it, I refer to this as the "speed loss dynamic". Any trucker knows that construction, traffic, hills and any other number of obstacles to maintaining speed causes this phenomenon; but try to discuss it with someone determined to reduce your truck's speed while continuing to demand sevice performance, and you will soon find that you are speaking to a deaf ear. Its simply too many details for people used to communicating in sound bites. I know that there must be a body of information on this subject beyond anecdotes and common sense. I have heard that the DOT in CA and elsewhere require drivers to log at speeds under the posted highway limits; this indicates that there must be something written somewhere that serves as the basis for such a policy. If anyone has some information on this subject, or knows where I can get some, please let me know. This subject most likely already has a label other than the one I have given it. Someone set me straight!!!