This paper describes a GIS-based environmental modelling system, termed TRAEMS, for use by transport planners in assessing the environmental effects of road traffic plans. The system utilises capabilities of GIS to integrate the output from a transport planning activity with land use information to model the environmental impacts of different road traffic scenarios. TRAEMS enables planners to test transport related environmental impacts at the same time as they are testing the traffic carrying efficiencies of network plans. The suitability of this type of environmental modelling for planning purposes is illustrated using a case study.