reliance is Andorra, which imposes no visa requirements of its own because it has no international airport and is inaccessible by land without passing through the territory of either France or Spain and is thus "protected" by the Schengen visa system. Visa-free travel between countries also occurs in all cases where passports (or passport-replacing documents such as laissez-passer) are not needed for such travel. (For examples of passport-free travel, see International travel without passports.) As of 2019, the Henley & Partners passport index ranks the Japanese, Singaporean, and South Korean passports as the ones with the most visa exemptions by other nations, allowing holders of those passports to visit 189 countries without obtaining a visa in advance of arrival. However, the Passport Index ranks the United Arab Emirates passport as the one with the most visa exemptions by other nations, allowing holders of this passport to visit 173 countries without obtaining a visa in advance of arrival. Common visas Normally, visas are valid for entry only into the country that issued the visa. Countries that are members of regional organizations or party to regional agreements may, however, issue visas valid for entry into some or all of the member states of the organization or agreement: The Schengen Visa is a visa for the Schengen Area, which consists of most of the European Economic Area, plus several other adjacent countries. The visa allows visitors to stay in the Schengen Area for up to 90 days within a 180-day period. The visa is valid for tourism, family visits, and business. The Central American Single Visa (Visa Única Centroamericana) is a visa for Guatemala, El Salvador, Honduras, and Nicaragua. It was implemented by the CA-4 agreement. It allows citizens of those four countries free access to other member countries. It also allows visitors to any member country to enter another member country without having to obtain another visa. Possible common visa schemes Potentially, there are new common visa schemes: An ASEAN common visa scheme has been considered with Thailand and the "CLMV" countries of Cambodia, Laos, Myanmar, and Vietnam opting in earlier. After talk arose of a CLMV common visa, with Thailand being omitted, Thailand initiated and began implementation of a trial common visa with Cambodia, but cited security risks as the major hurdle. The trial run was delayed, but Thailand implemented a single visa scheme with Cambodia beginning on 27 December 2012