I think that tax on petrol is a good way for the government to obtain revenue -- the money for public services has to come from somewhere! Petrol consumption inarguably damages the environment, so high petrol prices are a good way of discouraging excessive consumption e.g. by encouraging people to live near where they work, and looking at public transport, cycling, and walking. And although people moan about petrol prices, they obviously aren't high enough to have a significant effect on people's habits -- look at the number of 4x4s and other high-consumption cars that are about. Although I definitely feel that public transport and cycling should have money put into it.