A vehicle tumbled 100 feet down from Highway 243 above Banning, backing up uphill traffic towards Idyllwild and leaving a woman with major injuries.
Firefighters were dispatched at 11:18 a.m. to the road, about two miles south of San Gorgonio Avenue in San Gorgonio Pass, and found the mangled vehicle off the steep roadside.
A woman was flown out to a hospital for treatment of her injuries, Riverside County Fire Department spokeswoman Melody Hendrickson said.
Northbound traffic continued moving but slowed down when it reached the scene, the California Highway Patrol reported.
Southbound traffic on Highway 243 was completely stopped at the bottom of the hill, the CHP reported.
The highway is one of two routes into the mountain resort town of Idyllwild.