Agencies have the authority to require a traffic study be done according to their rules if the development is proposed to have an access from a road under their jurisdiction. A city almost allows has this authority because accesses are typically from city streets. Sometimes, an access is proposed on a county road and rarely is an access proposed on a state road.
Mn/DOT has a thorough policy for traffic studies that traffic engineers are supposed to follow if a development is impacting one of their roads/interchanges. Scott County also has a thorough policy, but it is hard to find on their website (here it is – Download 20051102 PDF Report from Scott County_Traffic_Impact_Analysis_Process_Report2).
Most cities in Minnesota don't have policies regarding traffic studies, but we've done some digging (and we know of a lot of them based on past experience). Here's a spreadsheet that lays out some of the local policies – Download Traffic Study Policies for MN Cities & Counties. Let me know if we missed any and I'll update the matrix.