The third segment, between Vasco and Marsh Creek roads, opened for public use on Sept. 30, program manager Dale Dennis said.
A 2 p.m. grand opening ceremony is expected to include Caltrans Director Will Kempton and U.S. Rep. Jerry McNerney, Caltrans spokesman Allyn Amsk said.
The bypass program was initiated in the 1980s, a response to increasing growth in the area and traffic congestion through downtown Oakley and Brentwood, Caltrans officials said.
The bypass was constructed to the west of those areas.
The opening of the third segment completes the first phase of the bypass program, Dennis said.
Further plans to add lanes and connector ramps along the bypass are estimated to cost another $145 million, he said.