Trails are an external memory, a form of collective intelligence. They save a traveler from having to discover a new route each time, instead offering a finite set of options that have proved valuable in the past. The wisom of a trail is demonstrated in its balance of durability, efficiency, and flexibility.

The discussion of trails in this book ranges from traces of ancient organisms, to ant paths, to elephant migratory patterns, to the Appalachian Trail. The Appalachian Trail could be regarded as one of the most successful human-made trails, as could the interstate highway system. Designing a sustainable walking trail, the author explains, requires managing the flow of water and the flow of people (whose behavior is not that different from water).

The role of technology in both traveling and making trails is also a recurring theme. Some modern tools, like water purifiers and GPS, are widely accepted by hikers, while others are spurned. In a way, the author argues, the internet itself is a system of trails represented by hyperlinks.

Understanding trails has important implications for emergent systems, such as swarm robotics. This quote, from a conversation the author had with Jean-Louis Deneubourg, is worth reproducing in full:

“The interesting thing is that in Rome, originally that was a grid system,” he said. “The whole ssytem was destroyed by time, and then it converged into a medieval organic system.” Likewise, many cities across Europe that were built on the Roman grid–Damascus, Mérida, Caerleon, Trier, Aosta, Barcelona–later collapsed back into an organic layout, as people began taking shortcuts across empty quadrants, filling in extravagant plazas, andaltering the imperial road network to their needs. Left to their own devices people unwittingly redesigned their cities precisely as ants would….

If he [Deneubourg] were the mayor of a new city being build ex nihilo, like Brasília, how would he organize it?

“I would like to see the emergence of a town,” he said. “If I was the mayor… my attitude would be very liberal. My objective would be to offer different types of material to help the citizens find the solution that they prefer…. To believe that you have the solution for another person is a form of stupidity.”

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