The crash was reported at 11:45 a.m. on southbound U.S. Highway 101 near San Antonio Creek where the construction is happening.
The driver was a 29 year old man. His father was in the passenger seat, and his mother was in the back seat. No one was wearing a seat belt, according to the CHP.
Part of the construction involves building a bridge over the creek.
"There was a 30 foot gap in the freeway that is yet to be completed. The car traveled off the paved portion and struck a bridge abutment wall," CHP Sgt. Jason Bahlman said,
"The driver was driving under the influence of both alcohol and drugs and the driver is under arrest," Bahlman said.
This is the second fatal suspected DUI in less than a week in Sonoma County.
On Tuesday, on Highway 12 in Agua Caliente, police arrested the driver of a pickup truck after they say he drove, head on into a car driven by a young mother.
She and her 7-year-old daughter died.
At that scene and at Saturday's, empty beer cans were found in the crashed cars.
The CHP has upped patrols and has all available officers on the roads this holiday week.