Construction Phases in Yerba Buena Tunnel / General Plan & Location of Bottleneck (1963)
In December, 1961, the former track lanes on the lower deck of the west span of the Bay Bridge were opened to general traffic, giving three lanes eastbound and two lanes westbound. In January, 1962, the right eastbound lane was designated for buses only. But there were still only four lanes through the tunnel while the partition was being removed and the upper deck lowered, so the two rightmost eastbound lanes had to merge together at the west side of the tunnel. It sounds like east of the tunnel, only one lane in each direction was available on the lower deck, and the traffic signal functioned like a metering light to merge the two eastbound tunnel lanes together into one.
From San Francisco-Oakland Bay Bridge Trans-Bay Bus Riders Survey, May, 1963.