Americans like to visit the national parks. In 1996, over 250,000,000 people visited areas which are managed by the National Park Service. Experts say that by 2012, 500,000,000 people per year will visit the parks. As a result, people are trying to think of ways to protect the parks from the crowds. The crowding of the parks has caused several problems. One is that there is not enough space for all of the people who want to use the parks. To deal with this problem, you may have to book your tickets a few months ahead of time. The Park Service may also raise entrance fees   . Another problem caused by increased park use is pollution. Some garbage and waste can be cleaned up, and the cleanup can be paid for by increased entrance fees. To cut down on noise pollution and air pollution, the number of cars allowed in parks may be limited. If this happens, people may use buses to travel around the parks. Officials may also limit the use of jet-skis, snow-mobiles, motor-boats, and sightseeing helicopters and planes. The parks may also be in danger because of development that is going on around them. For example, around Yellow Stone Park, many hotels, golf courses, resorts, and other tourist attractions have been built. Some of these developments have effect on areas that are used by animals. People will have to learn to agree about ways to protect the parks. If they do not, there will soon be no reason to visit these national treasures. According to experts, by 2012 the number of visitors   _  .
may be doubled may rise to 50% will reach 250 million will go up by a second