GOLDEN — After a week of collaboration, a team of construction experts presented their preferred option for improvements to the Twin Tunnels on Interstate 70 to help lessen congestion in the mountain corridor.

The recommended package has an estimated cost of $55 million and would widen the eastbound tunnel bore to three lanes. In addition, the 45 mph curve in the eastbound lanes just west of the Hidden Valley exit would be flattened to allow for 55 mph traffic.

During tunnel construction, eastbound lanes of I-70 would be detoured to the south on reconstructed frontage roads — the old U.S. 40 and a county road.

The package also calls for adding a third eastbound lane from the east end of Idaho Springs to Floyd Hill.

For this construction package, engineers propose to harden the shoulder of eastbound I-70 to allow temporary three-lane travel during peak traffic periods while construction is underway.

The team estimated the entire package would take four or five years to complete, with the actual widening of the tunnel bore taking as little as two months.

No funding has been identified for the project.,

Colorado Department of Transportation area director Tony DeVito said the next step is to present the preferred option, and six alternative packages, to the Colorado Transportation Commission next month.

Those alternatives include a $100 million proposal that would widen both bores of the Twin Tunnels to three lanes and flatten the curves in both directions on I-70 east of the tunnels. That option also would add an eastbound lane between Idaho Springs and Floyd Hill. The construction would accommodate a 55 mph speed limit from the tunnels east to the Hidden Valley exit.

Another option would do the same thing, but to standards that would allow a 65 mph speed limit at a $105 million price tag.

A third option would widen the eastbound bore of the tunnel to three lanes and flatten the curves on I-70 to a 65 mph standard. That proposal would cost $80 million.

Yet another proposal would realign eastbound I-70 on a bypass south of the tunnels on a 3-lane viaduct. It would cost about $65 million, the engineers said.

For the week-long “tunnel visioning” workshop, CDOT assembled 12 experts with special knowledge in tunneling, geotechnical engineering, roadway design, highway construction and traffic engineering. Each was paid $1,000 to attend the workshop and help come up with a short-term solution to congestion on I-70.

Jeffrey Leib: 303-954-1645

or jleib@denverpost.com