Sensitivity of Input Parameter on CNOSSOS-EU Road Emission Levels

Abstract

The United Kingdom Department for Food and Rural Affairs (Defra) commissioned a series of studies investigating the sensitivity of the CNOSSOS-EU noise assessment method. CNOSSOS-EU presents challenges in terms of input data accuracy and availability. For this reason, the studies were commissioned to support data decision making and quantify potential uncertainty in Defra's national noise model. The quality framework set out in Directive 996/2014 requires uncertainty in road emission levels at source to correspond to an uncertainty of ±2dB(A). By calculating emissions across a range of values for input parameter within various scenarios in which all other parameters were kept constant, changes in road traffic noise emissions were observed. This was performed separately for each of the five CNOSSOS-EU vehicle categories as well as for a selection of hypothetical composite road traffic flows. A similar analysis was then performed for a series of road surface types. The results were used to indicate which input parameters road traffic noise emissions were most sensitive to. The results identified that uncertainty in traffic flow speed has a tolerance of ±11 kmh-1 for cars and two-wheelers, and ±24 kmh-1 for HGVs in order to meet the quality framework. Significant variation in emissions were found across surface types, indicating the importance of obtaining representative road surface data. The study found that traffic volumes should be within 60% of their true value. The effect of road gradient was found to be dependent on flow composition and speed. Road traffic noise emissions were found to be insensitive to changes in air temperature. Composite flow analysis using English traffic statistics with various proportions of HGVs showed that these assumptions become more important on lower speed roads, while the proportion of two-wheelers is relatively insignificant with respect to overall noise emissions.