of movement. Imposition of an exit visa requirement may be seen to violate customary international law, as the right to leave any country is provided for in the Universal Declaration of Human Rights. Uniquely, the Norwegian special territory of Svalbard is an entirely visa-free zone under the terms of the Svalbard Treaty. Some countries—such as those in the Schengen Area—have agreements with other countries allowing each other's citizens to travel between them without visas. The World Tourism Organization announced that the number of tourists requiring a visa before travelling was at its lowest level ever in 2015. History In western Europe in the late 19th century and early 20th century, passports and visas were not generally necessary for moving from one country to another. The relatively high speed and large movements of people travelling by train would have caused bottlenecks if regular passport controls had been used. Passports and visas became usually necessary as travel documents only after World War I. Hitherto, passports and visas were usually the same type of travel documents. In the modern world, visas have become separate secondary travel documents, with passports acting as the primary travel documents. Conditions of issue Some visas can be granted on arrival or by prior application at the country's embassy or consulate, or through a private visa service specialist who is specialized in the issuance of international travel documents. These agencies are authorized by the foreign authority, embassy, or consulate to represent international travellers who are unable or unwilling to travel to the embassy and apply in person. Private visa and passport services collect an additional fee for verifying customer applications, supporting documents, and submitting them to the appropriate authority. If there is no embassy or consulate in one's home country, then one would have to travel to a third country (or apply by post) and try to get a visa issued there. Alternatively, in such cases visas may be pre-arranged for collection on arrival at the border. The need or absence of need of a visa generally depends on the citizenship of the applicant, the intended duration of the stay, and the activities that the applicant may wish to undertake in the country he or she visits; these may delineate different formal categories of visas, with different issue conditions. The issuing authority, usually a branch of the country's foreign ministry or department (e.g. U.S. State Department), and typically consular