In 2001 a metro system that had been in planning since the 1970s opened the first two of seven envisaged lines—even though the city is prone to earthquakes. Work has been slow and coverage remains very limited. Development of the Tehran metro system had been interrupted by the Islamic Revolution and the Iran-Iraq War. Problems arising from the late completion of the metro led to buses taking on the role of the metro lines, serving mainly long distance routes. Taxis filled the void for local journeys. The taxis only drive on main roads, and only within the local area, so it can be necessary to take several taxis to get to one’s final destination. This has all led to extreme congestion and air pollution within the city.