why does tehran metropolitan region (tmr) need a specific regional governance?

mega city-regions or metropolitan regions- that was introduced for the first time in a cabinet article in 1995- are becoming a dominant form of human settlements both in developed and developing countries. despite of all contentious discussion on economic and demographic significance of metropolitan regions and so rationality of establishing a distinct structure for their effective management and governance, there are a few institutions and governmental structures for effective metropolitan planning and governance. the main questions of present paper are: a. what are the economic, social, political and environmental rationality for establishing a specific structure or process for management and governance of metropolitan regions? b. what is the most important factor for explaining rationality of regionalism in tehran metropolitan region? according to literature review there are five arguments for explaining metropolitan regionalism: a. efficiency argument (economic rationality), b. strategic planning and effectiveness argument (planning rationality), c. democracy argument (political rationality), d. equity argument (social rationality) and, e. competitiveness argument (economic rationality). present paper  argue , by referring to study of spatial and political structure of tehran metropolitan region(tmr); three range of theories in metropolitan regionalism namely old regionalism, public choice and new regionalism, and also best practices in the field, that the most important argument on explaining rationality of metropolitan regionalism in tmr is arguments on enhancing policy effectiveness and creating strategic capacity dealing with regional environmental problems achieved by creating spatial strategic planning framework. present research argues that the most challenging environmental regional problem in tmr is sprawl. for doing detail research on this aspect, main theories presenting relationship between political fragmentation and sprawl was reviewed. research findings resulted from examination of spatial trends of regional growth in tmr confirm main sections of theoretical frameworks derived from works of known researchers in the fields. according to these theoretical frameworks, in the condition of political fragmentation, spillover growth always leaks from central and big cities (with exclusionary planning and lack of affordable housing) to unincorporated areas without growth control and cheap land available for development, the areas have great accessibility to major service centers. after a while and due to concentration of population in unincorporated area, these centers get incorporated and established municipality and growth control power. in the new established condition, spillover growth finds these new areas not desirable for development for their relative high cost and constraint on development comparing to unincorporated outer fringe. control of this process of sprawl which works in a cyclic manner and also carrying out growth management will not be possible in the condition of political fragmentation in which there is not any body for coordinated regional spatial planning. accordingly, the mission of any kind of regional body must be regional growth management. therefore, metropolitan regionalism (in the meaning of establishing specific structure for metropolitan governance) in tmr is mainly explained by referring to rationality of regional growth management (guiding growth and coping with region-wide sprawl), the mission which is impossible in the condition of political and functional fragmentation.