The graph illustrates the relationship between vehicle type and speed in determining nitrogen oxide (NOx) emissions, measured in grams per kilometre.
Overall, lorries consistently emit higher levels of nitrogen oxide than both passenger cars and buses across all speed ranges. While emissions from passenger cars decrease steadily as speed increases, emissions from lorries and buses display more fluctuation, particularly at lower speeds.
At low speeds (around 10 km/h), lorries produce the highest amount of nitrogen oxide, with emissions exceeding 30 grams per kilometre. In contrast, buses emit approximately 25 g/km and passenger cars release less than 10 g/km. As speed increases to about 60 km/h, NOx emissions from lorries and buses decrease but remain significantly higher than those from passenger cars, which continue to decline gradually.
Beyond 60 km/h, emissions from lorries and buses begin to rise again slightly, whereas passenger cars continue their downward trend, emitting the least amount of nitrogen oxide at higher speeds. Notably, even at optimal speed ranges, heavy vehicles like lorries still emit two to three times more NOx than cars.
In summary, vehicle type plays a major role in nitrogen oxide emissions, with lorries being the most polluting. Speed also affects emissions, particularly for heavier vehicles, but passenger cars consistently show the lowest levels of nitrogen oxide across all speeds.