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Statutory regulations over regional transportation modeling comes primarily from two sources: the Clean Air Act Amendments of 1990 (CAAA) and federal transportation law, ISTEA and follow-up legislation TEA-21, SAFETEA-LU and MAP-21. The transportation legislation works together with the environmental framework established by CAAA to reduce air pollution in urbanized areas, to monitor the progress made towards reaching clean air targets, and to facilitate future improvements in regional air quality by reducing emissions from mobile sources. In practical terms, the later goal has required regional planning agencies to demonstrate that their long-range transportation investment strategy will improve and not degrade urban air quality, which in turn has required an upgrading in forecasting abilities at the regional level.