Line graph — Nitrogen oxide (NOx) emissions from four types of vehicle

The line graph illustrates how much nitrogen oxide, in grams per kilometre, petrol cars, diesel cars, buses and lorries emit on average at speeds from 10 to 130 km/h.

Overall, emissions from all four vehicle types trace a U-shaped curve, being lowest at moderate speeds and highest at the two extremes. Buses and lorries are far heavier polluters than either type of car at every speed.

At 10 km/h the bus is the worst offender, emitting 11.8 grams against the lorry's 9.6. Both figures fall as speed increases and bottom out at 70 km/h, at roughly 5.6 and 5.9 grams respectively, which makes the lorry marginally the dirtier from this point on. Beyond that speed emissions climb again, and more steeply for the lorry, which reaches 13.1 grams at 130 km/h, 2.2 above the bus's 10.9.

The two cars follow the same pattern at a much lower level. Diesel cars produce 3.6 grams at the lowest speed, about 2 at 70 km/h and 4.3 at the highest, while petrol cars, the cleanest vehicles throughout, go from 2.1 to just under 1 gram and back up to 2.4. The diesel car thus emits roughly twice as much as its petrol equivalent. Even at its dirtiest, it stays below the lowest figure for either heavy vehicle.