Two vehicle crashes were causing traffic to back up on Highway 101 at Los Osos Valley Road in San Luis Obispo.
It was happening as the busy Friday evening commute was just beginning.
Both crashes happened at about 4:10 p.m. in the southbound lanes of the highway.
According to the California Highway Patrol, the fast lane was initially blocked as a result of one of the crashes. An ambulance was requested to respond. The second crash was determined to be non-injury.
Emergency crews were on the scene and traffic was backed up as much as three miles, according to Caltrans. Northbound traffic was also slow through the area.
Caltrans is suggesting Highway 227 as a detour.