  We consider an instance of a nonatomic routing game. We assume that the
network is parallel, that is, constituted of only two nodes, an origin and a
destination. We consider infinitesimal players that have a symmetric network
cost, but are heterogeneous through their set of feasible strategies and their
individual utilities. We show that if an atomic routing game instance is
correctly defined to approximate the nonatomic instance, then an atomic Nash
Equilibrium will approximate the nonatomic Wardrop Equilibrium. We give
explicit bounds on the distance between the equilibria according to the
parameters of the atomic instance. This approximation gives a method to compute
the Wardrop equilibrium at an arbitrary precision.
