Transportation and communication networks bring people together. Yet sometimes people themselves bring forth barriers to transportation and communication. In some countries, laws stop people from moving freely from place to place.
Over the centuries, many groups of people have been denied the freedom to travel because of their race, religion, or nationality. In the Middle Ages, for example, Jews were often forbidden to move about freely within certain cities. South Africa's government used to require black Africans to carry passes when they travelled within the country. Some governments required all civilians to carry identification papers and to report to government officials whenever they moved.
Countries set up customs posts at their borders. Foreign travellers must go through a customs inspection before they are allowed to travel in the country. Usually travellers have to carry special papers such as passports and visas(签证). Some countries even limit the number of visitors to their country each year. Others allow tourists to visit only certain areas of the country, or they may require that travellers be with an official guide at all times during their stay. Many of those barriers to travel also act as barriers to communication. When two governments have divisions on important matters, they usually do not want their civilians to exchange news or ideas freely. Countries often try to keep military or industrial information secret.
Today, people have the ability to travel, to communicate, and to transport goods more quickly and easily than ever before. Natural barriers that were difficult or dangerous to cross a hundred years ago can now be crossed easily. The barriers that people themselves make are not so easy to overcome. But in spite of all the different kinds of barriers, people continue to enjoy travel and the exchange of goods and ideas.
