This paper shows that monitoring campaigns in association with modelling, particularly detailed roadside modelling, are a useful and cost effective method of assessing air quality. It proposes that an approach based on detailed monitoring associated with concurrent meteorological data can be used in combination with a roadside model to assess concentrations at other locations near a road and in other weather conditions. Plans for improving air quality can also be assessed in simple ways. The need for complex models based on detailed emission inventories is avoided. Two examples from streets in London and Paris are presented in which measurements of CO, NOx and volatile organic compounds have been made. The measurements are interpreted using correlations between pollutant concentrations and with wind speed, and by using the USEPA CALINE4 model. It is demonstrated that air quality measurements can be used to assess the validity of road traffic emission factors