Construction work has begun on the $420m project in the US state of California to add a new two-lane fourth tunnel bore to the existing three Caldecott tunnels.

The new 3,389 feet tunnel will run through the Berkeley Hills and will feature 12-foot lanes, a 10-foot north shoulder, a two-foot south shoulder as well as seven emergency escape passages to the adjacent tunnel.

The project is expected to significantly ease traffic flow for motorists using the Caldecott Tunnel near Oakland and San Francisco.

The existing three tunnels on State Route 24 provide a total of six lanes for the heavy Bay Area traffic, serving an estimated 160,000 drivers daily.

Part of the project’s cost, $197.5m, comes from the state’s American Recovery and Reinvestment Act (ARRA) share of $2.54bn.

The project is also the country’s single largest investment of ARRA transportation funds to date.