Discussion
The upper outliers of figure \ref{255142} are aligned and thus probably correlated spatially. This may contradict the first interpretation saying that they result of simple errors. Looking at the map shown in figure \ref{737481} one notices directly that indeed the outliers coincide with the end of the highway arriving in Vernier. The cells of the grid with values superior to the median also correspond to roads with important traffic such as primary or secondary roads.