4.2. Differences in correlation by spatial units
The range of correlations between noise and air pollution metric increased by decreasing size of spatial unit from London Boroughs to 1 km × 1 km grid cells and neighbourhoods. The values of Moran's I confirmed that there were no spatial patterns in correlation values associated with neighbourhoods. Furthermore, we could not detect any associations between noise or air pollutants and traffic indicators, such as road density and vehicle kilometres that might explain the large differences in postcode level correlations between neighbourhoods. This is in line with previous research looking at associations between community noise and traffic-related air pollution. Gan et al. (2012), for example, did not detect stronger correlations between noise and traffic related air pollutants (PM2.5 and NO2) in areas close to major roads. Allen et al. (2009) did not find an effect of proximity to roads on correlations between noise and air pollution, comparable to our findings that correlations across postcodes close to road are of similar magnitude than across all of London.