The given graph illustrates the amount of nitrogen oxide emissions produced by four types of vehicles: buses, lorries, petrol cars, and diesel cars.
Overall, emissions from diesel and petrol cars remained almost constant across the speed range, while buses and lorries showed both downward and upward trends. Buses consistently emitted the highest levels of nitrogen oxide compared to the other three vehicle types.
At 10 km/h, buses produced about 40 g/km of nitrogen oxide, the highest value among all vehicles, while diesel cars emitted only around 6 g/km, the lowest. Emissions from diesel cars then remained stable at roughly the same level across all speeds. By contrast, emissions from buses decreased steadily until they reached a low of 25 g/km at 60 km/h.
Furthermore, emissions from petrol cars remained almost unchanged, fluctuating slightly between 10 and 11 g/km throughout the speed range. Lorries began at around 33 g/km at 10 km/h, then experienced a steady decline, reaching just below 20 g/km at 90 km/h. After this point, both buses and lorries recorded a gradual increase in emissions as speed rose to 130 km/h.