User‑optimized and System‑optimized Travel Behavior

Flows on physical networks, ranging from congested urban  
transportation networks to the Internet, are the result of users  
interacting with one another and the infrastructure. User‑optimized  
travel behavior and system‑optimized travel behavior result in  
entirely different flow patterns, with implications for congestion  
engineers and network designers. User‑optimized behavior (sometimes  
referred to as selfish behavior) corresponds to travelers selecting  
individually their cost‑minimizing routes of travel. In  
system‑optimized (unselfish) behavior, a central controller routes the  
traffic in a manner that minimizes the total cost to society. Under  
user‑optimized behavior, the addition of a new link to a network may  
increase the travel cost to every user. This counterintuitive  
phenomenon is known as the Braess paradox. We will discuss the  
relevance of this paradox to both transportation networks and the  
Internet, and demonstrate that the paradox may correct itself as the  
network?s demand increases.