Uploaded on 2014-11-04 by ThomasAlois

1. This is an image of an expressway in Singapore. The several lanes, lighting, and trees are all visible, physical features of the highway. 2. What is invisible is the volume of various types of traffic (cars, motor cycles, trucks, buses) during the day (e.g., the morning and evening traffic jams). Also invisible is the air and noise pollution generated by the traffic, and the protection from pollution that the greenery affords the adjacent residential areas. 3. The pollution levels along the highway can be measured (with and without trees). These measurements would allow planners to evaluate the environmental impact for various planning measures (more or less lanes, more or less green). [1]: https://edxuploads.s3.amazonaws.com/14150781004543258.jpg