Instead of mass transportation, resource-poor worlds often strove to develop "people mover" transporters, such as to bridge the distances between large cities, and these were sometimes involved in time travel crime. Because only the militaries in such societies had access to materials for experimentation, much academic research was theoretical only, and discovery of potential for time travel often did not include more complete understanding of it's dangers, and so many, especially initial or experimental people mover transporters had time travel functions included, according to the thinking that time travel might only duplicate non-living matter, and could therefore be used as a valid tactic to acquire more natural resources to eliminate the lack of ones vital for advanced technology, such as copper and petroleum.