There are two major domains in which photographers, much like all travelers, carry their gear on trips; “in-transit,” and during shooting. In Part I, below, I discuss the problems of “in-transit” travel, which affects how you pack for your trip. In Part II, I'll discuss bags for carrying your gear while “in-transit.” In Part III, I'll discuss carrying your gear while shooting.
Whether traveling by bus, train, ship or plane the issues of “in-transit” travel are similar. When traveling solely by car, the traveler is restrained only by the car's capacity and the traveler's personal convenience, so I'm won't concern myself with auto “in-transit” issues.
By “in-transit” I mean when you are traveling from home to a destination, between destinations, or returning home at the end of a trip.