The emission behaviour of road vehicles is regularly estimated by the use of emission factors and models. But there is only limited information available about the validity of such factors or models in real world situations. This is because validation requires roadside measurements and these add an additional element of uncertainty in the form of pollutant dispersion behaviour. Tunnel measurements can help to overcome this problem. Road tunnels can be used as big laboratories because the boundary conditions within such a tunnel are well known. Traffic flow can be recorded as well as air flow and the measured pollution concentrations can be compared to those based on the emission factors/models. Such measurements were undertaken in a 10 km long road tunnel in Austria for a period of six weeks during autumn 1998 and another four weeks during spring 1999. An analysis of the data showed that CO emissions derived from the measurements were in good agreement with the emission factors surprisingly, this was not the case for NOx (oxides of nitrogen), especially for NOx emissions from heavy duty vehicles. Due to the fact that the HDV share of the traffic was up to 35% of total traffic volume the effects of unbalanced emission factors were considerable. Differences of up to a factor of 1.6 between measured and calculated emission factors were recorded. (A)
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