The first segment rebuilt as a modern freeway was finished in 1947, stretching from Peninsula Avenue (then called Peninsular) at the San Mateo-Burlingame line to Broadway Avenue in Burlingame, including the overpass for Peninsula. Contracts in 1948 and 1949 extended the freeway north to South San Francisco, a distance of . This segment converted the existing two-way traffic on Bayshore Highway to a single three-lane one-way road and added a new three-lane roadway running parallel to it. The new roadway, intended for northbound traffic, was constructed east of the existing alignment, separated by a wide median, permitting a future expansion to four lanes in each direction. This six-lane freeway followed the existing highway to Broadway in Burlingame, but then took a more inland alignment past the airport at Mills Field (now SFO), and crossed the old road at South San Francisco, running just east of it to near the south end of the cut at Sierra Point. The routing had been adjusted to the west to allow an additional for airport expansion. Since this segment was being constructed on predominantly marshy land, of fill was taken from the arm of San Bruno Mountain that extended into the water as Sierra Point; the hill was essentially levelled and the remaining fill was used for airport expansion.