Congestion resulting from increased capacity in single-server queueing networks

In many networks for traffic and telecommunications, minimizing delays from entry to exit is a major concern of users. In user-optimal routing, each user chooses a path to minimize delay from entry to exit, given the existing paths chosen by all other users. Under user-optimal routing, at equilibrium all users experience the same delay. Many networks, especially data networks, are commonly modeled as networks of single-server queues. We report examples of single-server queueing networks with user-optimal routing in which adding servers or increasing the capacity of existing servers worsens the delay experienced by all users.