The construction of a new section of the M-40 ring road will give rise to a redistribution of traffic in the city of Madrid and its outskirts.

The use of a geographical information system (GIS) has provided a suitable instrument for assessing and locating traffic effects before and after the new infrastructure enters service.

Traffic information was used as basic input data for drawing up a pollution map, taking a standard day with unchanging weather conditions.

The model chosen used several parameters to compute the concentrations of pollutants at ground level.

Finally, the pollution map was cross-referenced with the standard maps in order to locate and quantify the population affected by the different pollution levels and the wildlife and historic and/or archaeological features at risk.

The assessment was made before and after construction of the new road.

The utilization of GIS methodology allowed the environmental changes to be automatically located with the use of a personal computer.

This proved to be an effective tool for carrying out environmental impact assessments.