Med Ship Man by Murray Leinster — Read

Calhoun considered coldly. They were beyond what had been the farthest small city on the multiple highway. They would go on past now-starlit fields of plants native to Maya, passing many places where trucks loaded with the plants climbed up to the roadway and headed for the factories which made use of them. The fields ran for scores of miles along the highway's length. They reached out beyond the horizon, -- perhaps scores of miles in that direction, too. There were thousands upon thousands of square miles devoted to the growing of the dark-green vegetation which supplied the raw materials for Maya's space exports. Some hundred-odd miles ahead, the small town of Tenochitlan lay huddled in the light of the distant star-cluster. Beyond that, more highway and Maya City. Beyond that --

Calhoun reasoned that the projector to make the induction cattle fence would be beyond Maya City, somewhere in the mountains the photograph in the spaceport building showed. A large highway went into those mountains for a limited distance only.