BRADENTON, Fla. — A plan to cut $500 million of costs from the $4.1 billion Louisville-Southern Indiana Ohio River Bridges Project has triggered the need for additional federal environmental studies.

The cost-reduction plan proposes changes that “have the potential to result in significant environmental impacts that were not evaluated” in the original final environmental impact statement, the Federal Highway Administration said in a notice Tuesday.

The long-proposed project would build two bridges and reconfigure the downtown Louisville interchange known as “spaghetti junction,” where I-64, I-65 and I-71 converge, by moving it south.

The FHA approved the project in what is known as a record of decision in September 2003.

Last month, Kentucky and Indiana officials announced plans to downsize elements of the plan by rebuilding the Louisville interchange in its current location, reducing the number of lanes, removing pedestrian and bike paths, and eliminating tolls on several facilities.

At the time, officials said they felt the changes would not require a new federal review.

But the FHA said Tuesday that the modifications would require a supplemental environmental impact statement, additional studies and review by a number of federal agencies, as well as public workshops and comments.

Chuck Wolfe, a spokesman for the ­Kentucky Transportation Cabinet, said the new studies do not represent a setback for the project.

“Both state sponsors believe the process can be completed and the record of decision amended by this time next year,” he said. “That would keep us on track toward the start of construction target date of August 2012.”

Under an aggressive schedule, the two states plan to present the draft supplemental environmental statement this summer. After a review by the FHA, there will be a public comment period, and officials said a final document could be completed by December.

The review, which also will consider the impact of collecting tolls on the new bridges, will take place as the Louisville-Southern Indiana Bridges Authority works toward developing a proposal to finance the project.

The bi-state authority, appointed last year, is studying various financing and construction options, including tax-exempt toll revenue bonds and public-private partnerships. ­

It is holding a two-day forum starting Wednesday to gauge private industry interest in the massive project and solicit additional cost-saving ideas in design, construction and financing.

The Ohio River bridges project was designated a national transportation priority during the administration of George W. Bush, partly because of the junction’s significance as a major north-south freight route.

With some federal and state funding over the years, Kentucky and Indiana have hired consultants that have completed portions of the project’s six design elements.

Consultants are currently conducting investment-grade traffic and revenue studies that include variable tolling options on behalf of the bi-state authority, which has bonding powers.

The strategies under consideration by the authority range from bidding out traditional construction contracts to using public-private partnerships to design, build, finance, operate and maintain the project, according to a financial update submitted to the FHA in December.

The finance plan is expected to undergo further refinement, though it is not clear what impact the cost-reduction plan would have on funding options.

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