system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do they decide the way mountain roads twist and turn?
gpt
Many of the mountain roads you may find started out as trails or paths. The path was whatever made sense to the people (or animals, as many started out as deer trails, etc) as the easiest way to get through the terrain. The path was then widened for horseback, then for carriages. Sometimes the paths were re-routed for fallen trees or washed out sections of the trial. Eventually these paths were improved with gravel or planks. When automobiles came along the paths were further improved and rerouted to make life easier on drivers. Houses and landmarks were established and the roads were routed to or around them appropriately. Eventually a new section is built for a particularly difficult or treacherous section. 

Some roads are a bit more deliberate than this, but many mountain roads you will find evolved over more than a century in one way or another.