  We consider a network pricing game on a parallel network with congestion
effects in which link owners set tolls for travel so as to maximize profit. A
central authority is able to regulate this competition by means of a (uniform)
price cap. The first question we want to answer is how such a cap should be
designed in order to minimize the total congestion. We provide an algorithm
that finds an optimal price cap for networks with affine latency functions and
a full support Wardrop equilibrium. Second, we consider the induced network
performance at an optimal price cap. We show that for two link networks with
affine latency functions, the congestion costs at the optimal price cap are at
most $8/7$ times the optimal congestion costs. For more general latency
functions, this bound goes up to 2 under the assumption that an uncapped Nash
equilibrium exists. However, in general such an equilibrium need not exist and
this can be used to show that optimal price caps can induce arbitrarily
inefficient flows.
