Due to the deterioration of several historic rock walls along Highway 50 at Echo Summit, a $4.9 million upgrade is planned for 2010.

On Tuesday, a 33-year-old woman plunged 600 feet down a mountain slope facing Christmas Valley when her Toyota Rav 4 went off Highway 50.

The woman, Chandra Sikaria from Folsom, survived. Preliminary investigation by authorities leans toward a suicide attempt, but her husband says it is “speculation.”

A spokeswoman for Washoe Medical Center in Reno said Friday that Sikaria is still hospitalized but would not give her condition. Authorities have said her injuries were not life-threatening.

A spot filled with sand bags in the highway’s barrier shielding motorists from the cliff was the entry point, authorities said. Such temporary fixes in a barrier, with stretches of rock quarried by inmates at Folsom State Prison in the 1930s, is common, and efficient, said Caltrans spokesman Mark Dinger.

The material helped earn the highway a historic designation.

Although the dry bridges, or viaducts, shaped with the Folsom stones can still deter wayward cars, Dinger said the material has deteriorated from weather and other adverse conditions. The nearly $5 million already earmarked in California’s 2009-10 fiscal budget will upgrade seven sections of retaining wall approximately one mile east from Echo Summit, Dinger said.

Reinforced concrete barriers an inch thick and 32 inches high will replace the existing barrier. The bottom portion of the retaining wall at road level will not be replaced. Turnouts for vistas will remain and drainage will improve. Dinger said the construction is expected to take two years, occurring in the dry months of Tahoe.

On average, a little more than 15,000 vehicles use the route on any given day, Dinger said.

John Rice is one of those motorists. As a resident of South Lake Tahoe and general manager for Sierra-at-Tahoe, Rice uses the route over Echo Summit daily.

“It’s certainly one where you have to keep your eyes open,” he said.

Rice has seen some cars hit the retaining wall but not go over the cliff. Sometimes people, at the same time, turn into the same small vista parking lot, creating a sticky situation for other drivers. One vista has large rocks where people sometimes step over the barrier and pose for pictures with Lake Tahoe in the background.

“I don’t think they understand what they’re dealing with,” Rice said. “I just sit there and hold my breath and hope nothing happens.

“I think it’s something some people take a little too fast, but for us it’s our lifeline,” Rice added.

Dinger said a similar smaller-scale project will occur along Highway 89 near Emerald Bay. Tentatively scheduled for next spring, a retention wall will be upgraded at a cost of $1 million.

Rice praised the efforts of Caltrans in its upkeep of the highway in both summer and winter but alluded the barriers might need an improvement.

“I doubt it would match today’s standards for road barrier but I prefer sandbags to orange cones,” Rice said.