Highway 41 between Morro Bay and Atascadero is down to a single lane of traffic after a crash involving a tanker truck.
As of 5 p.m. the truck — stopped near Morro Bay — was being prepared for towing.
Caltrans has placed electronic message boards warning motorists of the delay.
Drivers heading between the two cities can use Highway 1 to Highway 101 as an alternate route.
No information is available on how long traffic will be delayed along Highway 41.