While cars have undoubted advantages, of which their convenience is the most apparent, cars also have many significant disadvantages. Almost all cars run either on petrol or diesel fuel, both of which are fossil fuels. Burning these fuels causes the car to emit serious pollutants, such as carbon dioxide, carbon monoxide, and nitrous oxide. Not only are these gases harmful for health, causing respiratory disease and other illnesses, they also contribute to global warming, an increasing problem in the modern world. According to the Union of Concerned Scientists (2013), transportation in the US accounts for 30% of all carbon dioxide production in that country, with 60% of these emissions coming from cars and small trucks. Additionally, car ownership is increasing in almost all countries of the world, especially in developing countries, the amount of available roadway in cities is not increasing at an equal pace. This can lead to traffic congestion, in particular during the morning and evening rush hour. In some cities, this congestion can be severe, and delays of several hours can be a common occurrence. Such congestion can also affect those people who travel out of cities at the weekend. Spending hours sitting in an idle car means that this form of transport can in fact be less convenient than trains or airplanes or other forms of public transport.