More than half of the world's population live in cities. Traffic and pollution are becoming big problems in big cities all over the world. In cities like Mexico City, Shanghai or Cairo the quality of air is getting worse every day. In the United States, people spend more time sitting in traffic jams. Some of the world's cities have already found the answer to the problem. In Bogota, the capital of Columbia, the government has taken measures to improve the quality of life for the city's 8 million people. They created over a thousand new parks, a new public transport system, and built hundreds of cycling and walking paths. People who live in the city also joined in making their city one of the most livable   places in South America. Today city planners around the world are looking for ways to provide more room for living and less room for cars. In America, public transport has increased over 2 % since the mid 1990s -- not much but a start. More and more people leave their cars at home and get on buses, trains or even ride a bike to work. Some cities are far better at planning than others. In Amsterdam, for example, only 40 % of the population use their cars to get to work, 35 % ride bikes while 25% use public transport. In Paris, about half of the city's workers drive their cars to work. For cities in developing countries, the problems have just started. Traffic is getting worse as more and more people can afford to buy a car. The writer listed Mexico City, Shanghai and Cairo as examples of cities of   _  .
heavy traffic air pollution rapid development large population