Three alternatives to the controversial Scheme Z highway project were unveiled at a press conference Friday where the Central Artery/Tunnel project also announced that drafts of environmental impact reports for each plan had been filed with the state.

Scheme Z, a 16-lane, 11-story structure slated for construction in East Cambridge, was approved by then-Massachusetts Environmental Affairs Significant opposition from those in the Cambridge community fearing increased pollution and traffic.

One of the alternatives presented at the press conference, 8.ID Mod 5, had already won approval from the Cantabrigians who had opposed Scheme Z. Its plan calls for a 10-lane bridge across the Charles River and a three -lane tunnel under the river.

But 8.1D Mod 5 has an estimated cost of $1.28 billion, $539 million more than the estimated cost of Scheme Z. And last September, the U.S. Army Corps of Engineers criticized the environmental impact of that plan's river tunnel.

"Many people have expressed concern about the project's Charles River crossing." said Central Artery/funnel project director Peter M. Zuk in a statement released Friday. "The public now has an opportunity to review and to comment on the four designs under construction."

The other two alternatives unveiled at the press conference would cost less than 8.1D Mod 5 and would take less than the 13 years estimated necessary to build 8.1D Mod 5, according to state officials.

The reduced river-tunnel plan, estimated to cost $1.13 billion over nine years of construction, shrinks the proposed river tunnel from three to two lanes and reduces its length by more than one third. It would add two loop ramps on the north side of the river and would have a 12-lane bridge.

The non-river-tunnel alternative, the third plan, consists of two bridges with a total of 14 lanes crossing the Charles River but no river tunnel. It is estimated to cost $995 million and estimated to take 8.5 years to build.

"In choosing among the four possible designs, we must weigh a variety of competing interests," James J. Kerasiotes, Massachusetts secretary of transportation, said Friday.

"We will carefully analyze the environmental and aesthetic impacts of each option and balance those factors against traffic operations, safety and cost before selecting a final design," he said