A good understanding of the acoustic environment due to traffic in urban areas is very important and can be obtained by long term model based monitoring within large areas. As a result one has a good basis for assessing the effect of mitigating measures and for communication and policy making. A monitoring approach has been developed, based on two types of low cost acoustic sensor nodes; a basic wireless sensor node and an advanced sensor node. The basic node measures sound levels, while the advanced node measures time signals and is programmable to compute sound properties like the spectral content. Both sensor types can be freely combined in an acoustic sensor network. The network is scalable and is built up of subnetworks. Each subnetwork is self organizing and the data from the basic nodes within each subnetwork is sent to an advanced sensor via multi-hopping. Each advanced sensor node can be accessed by a remote connection and sends the data regularly to a central database. The acoustic sensor network is tested in an urban environment. It will be shown that reliable traffic noise data can be retrieved from the monitoring data, which can be used for the aforementioned purposes.