The paper presents a method of traffic analysis for a network which has extensive alternative routing and mutual overflow. The model is based on the mean and variance of the traffic and uses the Bernouilli-Pascal-Poisson (BPP) procedure to describe individual traffic streams on each link of the network. The method is applied to product graph networks where the load on each link includes a large number of traffic streams resulting from one or more stages of overflow. The node-to-node blocking probabilities and other relevant parameters are computed and compared with values obtained from a simulation study, and with values resulting from a simplification based on the equivalent random traffic (ERT) method. The model is also used to calculate the sum of the covariances between the marginal streams of traffic carried on individual links.