Miami-Dade’s expressway authority has launched a multi-million-dollar study of what could be one of the most controversial transportation ideas to come down the pike in a while: A new north-south expressway that could run through farms and ecologically sensitive wetlands at the county’s extreme urban edge.

The most likely route, according to a conceptual plan developed by MDX, would take it along Krome Avenue, meaning the multi-lane expressway — billed as a “southwest extension” of State Road 836 — would run almost entirely outside the county’s Urban Development Boundary, on land now zoned for agriculture or environmental conservation.

MDX officials, who inked a contract for a five-year, $6.9 million study of the idea, say the expressway would extend south to Southwest 136th Street, providing relief for Miami-Dade’s sprawling and traffic-choked far western suburbs.

But environmentalists, planners and urban advocates have slammed the concept as an outdated car-centric scheme that will do little to relieve congestion, but instead open up tens of thousands of acres of farms and plant nurseries beyond the UDB to further suburban development. The contemplated roadway would also bring pollution and development closer to the imperiled Everglades National Park, they say.

“We think there is no need for this extension. It poses a direct threat to the Everglades and to the Redland,’’ said Jonathan Ullman, Everglades field organizer for the Sierra Club in Miami. “It’s just a sprawl road designed to benefit sprawl developers.’’

Administrators at MDX, a semi-independent agency that manages the county’s toll highways, say they’re well aware of the area’s environmental sensitivity. But they say many of the 850,000 residents along the corridor must now rely on dangerous Krome Avenue to travel north and south by car. Existing expressways to the east, meanwhile, are virtually inaccessible because of congestion on local roads, MDX executive director Javier Rodriguez said. And further growth is expected inside the urban boundary, he noted.

“Talk to the residents who live out there. It’s absolutely frustrating and devastating to go east,’’ Rodriguez said. “It’s crazy what’s happening out there. We have to deal with an existing problem that we have. How are going to deal with the growth?’’

The dilemma, some critics say, is the direct result of Miami-Dade’s permitting of scores of car-dependent, single-family home subdivisions far from major job centers in an environmentally sensitive zone — once all part of the Everglades watershed — without adequate roadways or mass transit.

But it’s unclear whether MDX’s concept will prove doable in an era of shrinking budgets for highway building and maintenance, and growing skepticism over whether highway expansion relieves congestion, or simply engenders more.

MDX has been quietly laying the foundation for the expressway for years. The agency last month signed a deal with Stantec, a multinational planning and engineering firm, to study the feasibility, environmental impact and best potential routes for the possible expressway. But the idea was first outlined in a 2009 conceptual plan for expansion of the agency’s network of toll roads. The new study will include substantial public input, a schedule of public events and the establishment of citizen advisory committees, MDX says, but it doesn’t mean the road will get built.