Overall, a general trend can be observed in which nitrogen oxide emissions initially decreased as speed increased before rising again at higher speeds. Meanwhile, buses and lorries consistently produced far higher emissions than petrol and diesel cars. It is also noticeable that buses recorded the highest emission levels across all speed
At the start of the speed range, the figure for buses was about 40 g/km, compared with roughly 33 g/km for lorries. As speed increased, emissions from buses fell steadily, reaching approximately 27 g/km at around 60 kph. In contrast, lorries also showed a downward trend, declining to about 23 g/km at the same speed.
With regard to the other categories, petrol cars experienced relatively low emissions, moving from around 12 g/km to about 10 g/km before rising slightly at higher speeds. Conversely, diesel cars changed differently, as their emissions remained the lowest and relatively stable, fluctuating between 5 and 7 g/km. Overall, the data clearly compares emission levels across vehicle types and speeds.