We consider the problem of routing traffic to optimize the performance
of a congested network. We are given a network, a rate of traffic
between each pair of nodes, and a latency function for each edge
specifying the time needed to traverse the edge given its congestion;
the objective is to route traffic such that the sum of all travel
times---the total latency---is minimized.
In many settings, it may be expensive or impossible to regulate
network traffic so as to implement an optimal assignment of routes.
In the absence of regulation by some central authority, we assume that
each network user routes its traffic on the minimum-latency path
available to it, given the network congestion caused by the other
users. In general such a ``selfishly motivated'' assignment of traffic
to paths will not minimize the total latency; hence, this lack of
regulation carries the cost of decreased network performance.
In this paper we quantify the degradation in network performance due
to unregulated traffic. We prove that if the latency of each edge is
a linear function of its congestion, then the total latency of the
routes chosen by selfish network users is at most $\frac{4}{3}$ times
the minimum possible total latency (subject to the condition that all
traffic must be routed). We also consider the more general setting
in which edge latency functions are assumed only to be continuous and
nondecreasing in the edge congestion. Here, the total latency of the
routes chosen by unregulated selfish network users may be arbitrarily
larger than the minimum possible total latency; however, we prove
that it is no more than the total latency incurred by optimally
routing {\em twice} as much traffic.