Do you live in a city or have you visited a city close to where you live? If so you will no doubt be familiar with the variety of vehicles on our roads (Figure 7.8). Some are small cars, others are heavy motor trucks. These vehicles differ not only in their size, but also by using different types of fuel such as petrol, diesel, and blended fuel (10% ethanol and petrol). If you observe the tailpipe of diesel engine vehicles, you will have seen the black exhaust gas produced. The intensity of the black colour is greater for poorly maintained vehicles, to the extent sometimes that it makes the air hazy or smoky and causes coughs and eye irritation. The lack of a policy to remove old vehicles from the roads adds to the problem. Tiwari (2012) found that nearly a third of vehicles in Addis Ababa were over 30 years old, resulting in high levels of tailpipe emissions. Traffic jams, common in all big cities, make the problems worse.