Culturally the Benelux countries are interesting for their heritage of historic buildings and art treasures, a reminder that the region has played a major role in European history. However their economic prosperity has frequently led to conflict with powerful neighbours, with the result that after the Second World War Belgium, the Netherlands and Luxembourg led the way to European unity with the formation of a customs union. This means that restrictions on movement between the three countries are minimal. With a combined population of 27 million, the Benelux states are the most densely populated countries in Europe. Not only does this lead to intense competition for land use, but it also places pressure on the environment to the extent that any proposed tourism developments are very closely scrutinized. The economies of the three countries have grown steadily since the Second World War, giving rise to increasing demands for both domestic and foreign tourism. Expenditure on overseas travel exceeds the receipts from inbound tourism in all the countries of the region. Annual holiday entitlement averages five or more weeks and a typical working week is less than 40 hours.