Bangkok's rapid growth amidst little urban planning and regulation has resulted in a haphazard cityscape and inadequate infrastructure systems. Limited roads, despite an extensive expressway network, together with substantial private car usage, have resulted in chronic and crippling traffic congestion. This in turn caused severe air pollution in the 1990s. The city has since turned to public transport in an attempt to solve this major problem. Four rapid transit lines are now in operation, with more systems under construction or planned by the national government and the Bangkok Metropolitan Administration.