Many experimental studies report disappointingly low throughput along multi-hop wireless CSMA/CA networks. This bad performance is not only due to MAC protocol issues, but to the subtle combination of the MAC layer dynamics and the packet dynamics along the path. When the density of packets is too high, self-interference occurs between packets, and the overall throughput drops. This behavior is very similar to that observed on congested roads. We propose a Markov model that captures this phenomenon, and show the similarities with models in road traffic engineering. In some particular case, we compute analytically the throughput of the congested mode.