The bore, which is expected to unclog off-peak traffic heading toward the tunnel, opened around 4:25 a.m., with CHP crews leading the first cars from Orinda to Oakland, the CHP said.
Crews will continue to work on realigning bores 1 and 2, leading up to Bore 3 for motorists to experience a smooth transition through the tunnel, the CHP said.
All four bores are expected to be open around 8 a.m. today.
The new bore will create a smoother flow of traffic for the more than 160,000 people who travel through the Caldecott Tunnel each day by keeping four lanes of traffic open in each direction at all times, Caltrans officials said.
The additional tunnel will allow Caltrans to end its 50-year-old practice of manually reversing the flow of traffic twice per day using the middle bore.
Drivers heading in off-peak directions leading up to the tunnel, or east on weekday mornings and west during the evening rush hour, could shave an average of five minutes from their morning commute cut an average of 7 to 10 minutes from the westbound evening return trip, according to Caltrans.
Weekend travelers heading in off-peak directions will also have more predictable traffic approaching the tunnel thanks to the additional two lanes, Caltrans officials said.
The fourth bore is also outfitted with a number of safety upgrades, including wide shoulders, a series of powerful jet fans to blow smoke out of the tunnel in the event of a car mishap or fire and cross-passages connecting the third and fourth bores that serve as emergency exits.
The $417 million project was funded mostly by federal stimulus funding as well as through revenue from Measure J, a half-cent sales tax approved by Contra Costa County voters in 2004. Funding also came from Bay Area bridge toll revenue, and proposition 1B, a 2006 voter-approved state transportation bond.
The project received one of the greatest shares of funding under the American Recovery and Reinvestment Act and created more than 5,000 jobs, according to Caltrans.