Fairy Paths are used as trade routes and highways, they are enspelled to keep travellers inside and would-be brigands from entering. It is almost exclusively used by royals.
The path appears from the inside as a spectral green mist, but from the outside only the travellers can be seen. Inside the path one can only move along the trail, attempts to step outside only loop back. It exists on another plane, therefore there is no "outside", despite what the visual terrain may have you believe.
Taking a Fairy Path greatly cuts travel time, but if you don't have the "key" to escape you will be lost on the trail for quite some time. Unless you have a coat to turn inside-out, that is! This is not something the fairies wish to be widely known, though. Better to have people believe they won't get away with Robin Hood antics.
The concept was coopted from Pixies, who used enspelled patches of grass to get travellers hopelessly lost for the sheer mean-spirited enjoyment of it. It was also used as a deterrent to keep mortals from blundering onto Faerie lands, especially greedy folk looking to steal riches from the bountiful and decadent denizens of fairy mounds. Those who steal from the fey certainly get their just desserts, but sometimes a mortal is too clever...
It is every merchant's dream to get a boon from the Fairy Courts to use their path to travel between markets, but this is a feat rarely achieved and just as hard to maintain. Only the richest of caravans can keep up with demand, and end up giving away a fair bit of their stock just to keep the privilege.