A sustainable transportation system is one that is safe, capable and friendly to the environment. Sustainable transportation is about combining economic, social and environmental factors into decisions that affect transportation activity. In economic aspect, we need a transportation system that is efficient. In social aspect, our transportation system must be safe and easy to be reached. In addition, we need a transportation system that respects the natural environment. It is not always easy to balance these three factors but there are also win-win-win opportunities. Sustainable transportation is important. Although transportation brings many economic and social benefits, the movement of people and goods can have important influences on the environment. These influences can in turn have social and economic consequences. Sustainable transportation calls for ensuring that the environment is considered along with economic and social considerations in transportation decision-making. Environmental influences of transportation include air and water pollution, greenhouse effects, and the use of land and other natural resources. A range of transportation activities contribute to these pressures, including the construction of roads; the production, operation, and throwing away of vehicles; and the supply of energy and fuel. A major challenge of sustainable transportation is to control or prevent air pollution and greenhouse effects. What is the main idea of the passage?