The Texas Department of Transportation has unveiled an $8 billion expansion plan for Interstate 35 that would add lanes to the highway through Austin and result in the construction of two levels of underground tunnels near the University of Texas.

TxDOT made the presentation Monday night during a board meeting of the Capital Area Metropolitan Planning Organization, which voted to kick in $400 million for the project in hope of enticing state and federal transportation agencies to follow suit.

"This is a huge deal," Austin Mayor Steve Adler said. "This is the regional (transportation) project that is our singular most important."

Marisabel Ramthun, TxDOT's director of planning and development for the project, stressed that the plan is preliminary. It likely will change substantially as the agency studies the project, reaches out to the public and conducts engineering studies.

As it stands, the project would add to the interstate one managed lane each way for the 12 miles from Texas 45 North in Round Rock to U.S. 290 East in North Austin.

Managed lanes can come in many forms, including high occupancy vehicle lanes. The new lanes will be separated from other free-flowing traffic, similar to the MoPac toll lanes, but the I-35 managed lanes will not be tolled, Ramthun said.

Two managed lanes would be added each way for 16 miles to the south, from U.S. 290 East to Texas 45 South.

The plan calls for dramatic changes to the interstate in downtown Austin. TxDOT's animated presentation showed two levels of tunnels in the stretch from Airport Boulevard to Martin Luther King Jr. Boulevard.

Five lanes each way would be on the first tier, with two managed lanes each way placed below. Frontage roads would be at ground level. From Martin Luther King Jr. Boulevard to Cesar Chavez Street, the highway would be lowered below ground level, but would not travel through tunnels.

The upper level bridges of I-35 that now stretch from 15th Street to 51st Street would be eliminated.

TxDOT has ranked four stretches of I-35 in the Austin area as being among Texas' most-congested roads. The 8 miles of interstate between U.S. 290 East and Texas 71 ranked as the third-most congested stretch in the state.

CAMPO's board voted unanimously to add the project to its regional transportation plan. The board of elected officials from the region's six counties are tasked with creating a 25-year mobility plan that's eligible for federal money.

The hope is that by allocating $400 million to the project, CAMPO shows the Texas Transportation Commission that it is serious about I-35's expansion. That could entice the state commission to kick in a few billion dollars, which might, in turn, lead to some federal money to help cover the amount needed to fully fund construction.

Ramthun's presentation showed construction beginning in 2022, with projected completion sometime after 2027.

The CAMPO board also allocated $75 million to construct frontage roads on US 183-A from RM 1431 to Avery Ranch Boulevard and $25 million for upgrades to the intersection of RM 620 and Anderson Mill Road.