While Gov. Chris Gregoire and state legislators will have the final say in the design of the new State Route 520, mediation efforts between area community leaders to endorse a West-end interchange plan are reportedly going well, with two of the three finalist concepts picking up speed, said Ron Paananen, a Washington State Department of Transportation official.

Of the original 12 designs, three remain: A, K and L. All three would implement similar features but offer different solutions for handling cars and buses on the West end of the proposed six-lane 520 corridor. Plans A and K have gained the most backing from the team members involved in the mediation process.

“Option L is one that does not have a great community constituency at this point,” Paananen said.

Representatives from Seattle neighborhoods, the UW, Metro and Sound Transit, the Arboretum Foundation and the Arboretum and Botanical Garden Committee, among others, are involved in the process of pinpointing a West-side design.

“We are hopeful at this point that we can reach something close to consensus,” Paananen said. “But there are a lot of issues between the different communities on [the West side] — very challenging.”

Mediation team members are supposed to reach an agreement and come up with a project impact plan by December. These will be submitted to Gregoire and the Joint Transportation Committee of the State Legislature.

Officials from the WSDOT say they want a new bridge in place for drivers by 2014, with a six-lane corridor opening in 2016. The projected cost of the SR 520 project ranges from about $3.7 billion to $3.9 billion and would be partially covered by tolls.

Plans A, K and L would all add lanes across or beneath Montlake to reduce congestion, narrow the structural footprint in Portage Bay, avoid Marsh Island at the Arboretum, relocate the Museum of History & Industry, improve storm water facilities, create new bike and pedestrian paths and restore neighborhood connections, among other changes. Yet there are major differences among the three alternatives.

Matthew Stubbs, president of the Eastlake Community Council, said members of the council have yet to take an official stance on which of the three alternatives they will support but will make a decision by mid-July.

“We will continue to weigh the environmental impacts, the proposed footprint and the concerns voiced by other communities and interest groups as we make our decision,” he said.

Theresa Doherty, UW assistant vice president for regional affairs, said the UW isn’t officially endorsing an interchange design yet, but certain pieces of concept A do appeal to University officials.

Alternative A would keep the interchange as it is now, while splitting north- and south-bound traffic on Montlake Boulevard with a second bridge. It would also prioritize ramps for bus use only and eliminate Lake Washington Boulevard ramps to the Arboretum and East Madison Street.

The Parkway Plan, also known as alternative K, calls for tunnel construction between McCurdy Park, the Montlake Cut and the Husky Stadium parking lot. It would also move Lake Washington Boulevard ramps to the west and add a green-space lid between where the tunnel, Montlake Boulevard and Pacific Street would meet. The proposed construction would have the smallest footprint through Portage Bay.

Doherty told The Daily in April that she was concerned about alternative K because construction would take three to five years and require 15 acres of land for the tunnel. She also worries about pedestrian safety on the intersection of Montlake Boulevard and Pacific Street.

“This will adversely impact the people who use the Medical Center parking lot on a daily basis,” she said. “We are also concerned with what this will do to the stadium, as well as the long-term increase in traffic in areas around the stadium and local neighborhoods.”

Paige Miller, the Arboretum Foundations executive director and mediation representative, told The Seattle Post-Intelligencer that the boating, bicycling, Arboretum, Madison Park, North Capitol Hill, Portage Bay, Roanoke, Montlake and Laurelhurst communities are supporting alternative K. Miller could not be reached for comment at print time for this story.

The third and seemingly least-favored plan, L, calls for an angled bridge from the Montlake Cut to the UW campus, green-space lids and Lake Washington Boulevard ramps.

About 350 people attended two open houses last week — one in Bellevue and one in Seattle — to hear the latest results from the mediation process.

“It was very well-received,” Paananen said. “People were very happy with the way we were conducting the mediation process and the transparency and how visible things are right now.”