Despite an extensive road and rail network, most long-distance travel within Europe is by air. A large tourism industry also attracts many visitors to Europe, most of whom arrive into one of Europe's many large international airports. London is the second busiest airport in the world by number of international passengers, only trailing Dubai. The advent of low cost carriers in recent years has led to a large increase in air travel within Europe. Air transportation is now often the cheapest way of travelling between cities. This increase in air travel has led to problems of airspace overcrowding and environmental concerns. The Single European Sky is one initiative aimed at solving these problems.