Three lanes were blocked Wednesday on southbound U.S. Highway 101 just north of East Poplar Avenue in San Mateo, according to the California Highway Patrol.
The collision occurred around 4:40 a.m. and involved a vehicle that crashed into the center median.
A Sig-alert was issued at 5:29 a.m. There is no estimated time of reopening the lanes, according to the CHP.
No further details were immediately available.