Do you know that you can report 'smoky vehicles' in Queensland to the Department of Transport and Main Roads (DTMR)? I find this fascinating.
Basically the premise is that smoky cars are a pollution source. DTMR states on their website that if you see a car smoking for more than 10 seconds continuously you can report the car (make, model and other details as described on the above webpage are required) to the Department. Apparently the owner of the car is then sent a letter explaining ways to fix the car and if the car is reported again within a fairly short period of time they will be required to present their car to an inspector to be checked for defects.
Now I will have to look into what the pollutants that a vehicle that is smoking may be contributing to the air and the significance of such pollution.