I speak about traffic for the purpose of illustrating the more general point of socially optimal energy use, and I restrict myself to the locomotion of persons, including their personal baggage and the fuel, materials, and equipment used for the vehicle and the road. I purposely abstain from the discussion of two other types of traffic: merchandise and messages. A parallel argument can be made for both, but this would require a different line of reasoning, and I leave it for another occasion. AUTHOR'S NOTE: This note appeared in the original text. I was then preparing two studies that were to complement this text: one on the history of mail delivery, the other on crews and loads throughout history. I renounced both projects to write Medical Nemesis.