Roadways, similarly, should be used, but those that are not commonly used should not be maintained, and those that are found to be needed should be maintained with concrete (a less limited resource than asphalt, which is oil based) by machinery operated as minimally as possible, and, ideally, replaced with alternate fuel source machines. The roads should be traveled by vehicles that either minimize resource use (like hybrid cars) or use renewable energy sources (solar cars, horse-drawn buggies, etc).

This need for the use of existing infrastructure indicates that any method for population reduction that would result in large-scale destruction of that infrastructure (i.e. thermonuclear war) are not acceptable.

This brings the question to mind of how best to institute this policy. The process needs to eliminate a large, but controlled, portion of the world’s human populace. This would point to biological, biogenic, chemical, or thermonuclear warfare. The result needs to be short term- people need to be able to populate the affected areas again within weeks of the event. This eliminates the options of biological warfare and thermonuclear approaches. Naturally occurring biological organisms have too high a mutation chance, which could allow them to spread beyond the original project scope. Thermonuclear war of course leaves radioactive residues that take centuries to dissipate. It also needs to leave infrastructure in place. This eliminates thermonuclear war again, and also certain forms of chemical warfare, as these attack buildings and make them structurally unsound. Finally, it needs to leave the ecosystem undisturbed to as much extent as possible. This eliminates biological warfare, since the organisms have the tendency to mutate, and may cross hosts from human, and chemical warfare of all types, since all effective chemical warfare agents react with all organic elements, regardless of species.

It would appear then that biogenic warfare, using specifically tailored biological organisms that target humans exclusively, is the best choice. Finally, as mentioned before, no biological weapon has a 100% effectiveness rate. Therefore it would be impossible to eliminate the entire population. It should also be considered what a sufficient genetic density in the world would be. If immunity rates are 1 in 10,000, this leaves a minimum of 600,000 people- easily enough for a genetically sufficient population.