A Caltrans subcontractor performing some trenching work punctured a steel pipeline 2 inches in diameter around 8 a.m., San Mateo police Sgt. David Norris said.
Norris said contractors working along the 300 block of South El Camino Real in the city's downtown area flagged down a San Mateo Fire Department engine company to report the break.
Fire officials immediately contacted police, and the two agencies began evacuating two buildings -- the Bank of America at 300 S. El Camino Real and the U.S. Bank at 136 Second Ave. -- and sent a Rapid Notify alert to all homes and businesses in the immediate area advising them to shelter in place, Norris said.
PG&E crews responded and shut off the gas supply by 10:54 a.m., a little more than two hours after the subcontractor hit pipeline, according to PG&E Spokeswoman Brittany Chord.
El Camino Real was shut down between Third and Fourth avenues and traffic was re-routed with the help of Burlingame police, Norris said.
PG&E crews dug a pit on the northbound side of El Camino to allow gas to seep out, and will work to repair that area and the line throughout the afternoon, Chord said.
There were no injuries, Norris said.