In the littoral region near Barcelona, roads belonging to the national network go through towns. Among the problems that this situation causes on the urban quality of ltfe is a high degree of noise pollution. By-pass roads (under construction) will modtfy acoustical maps of medium-sized cities by changing all circulation itineraries inside the city and increasing urban ltfe quality by reducing noise pollution. As an example, we present our study carried out in Matard, a city located 30 km from Barcelona on its north side. The high trafic noise is compared to the predicted acoustic level when a new by-pass road comes into operation.