OAKLAND  (AP) — The long-awaited fourth bore of the Caldecott Tunnel opened early Saturday to motorists traversing the often congested roadway between Alameda and Contra Costa counties.

The new bore, which runs under the Oakland Hills, opened to traffic around 4:30 a.m. Drivers now have four permanent lanes in each direction when they drive on Highway 24 between Oakland and Orinda.

Since the third bore opened in 1964, transportation officials had switched the direction of the middle bore twice a day to accommodate the heaviest traffic. That led to major congestion for drivers heading in the direction with only one bore available, because they had to merge from four lanes to two lanes.

The fourth bore will mainly help motorists driving in the reverse commute direction — eastbound in the morning and westbound in the afternoon — who previously could only use one bore of the Caldecott Tunnel.

“For people doing the reverse commute, it should be nice, right?” Daniel Tuggle, 41, an Orinda resident who drives to his advertising job in San Francisco, told the San Francisco Chronicle. “And it should help on weekends for people headed into the city or coming back.”

Officials say construction of the new bore, which began in 2010, was complicated because of changing soil conditions and the presence of potentially explosive pockets of methane.

“The project was incredibly complex,” Randy Iwasaki, executive director of the Contra Costa County Transportation Authority, told the newspaper. “We never knew what we were going to encounter.”

The fourth bore is wider, taller and brighter than the other three holes. It features a 10-foot-wide shoulder and walkways, and it’s equipped with modern ventilation, air and traffic monitoring systems.

Money for the $417 million fourth bore included $194 million from the federal American Recovery and Reinvestment Act, $124.6 million from a Contra Costa County tax measure and $50 million from Bay Area bridge toll funds.