STANFORD -- A pre-dawn accident on Interstate 280 claimed the lives of three popular Webb Ranch "school horses," while sending two people -- drivers of the cars that hit them -- to Stanford Medical Center for treatment.

The ranch, adjoining Portola Valley on land leased from Stanford University, has been in operation for 90 years and has been a training ground for generations of young equestrians.

"We lost three horses; that's a terrible shame, and all the children that take lessons will be very sad when they find out. But it wasn't human life," said Tom Hubbard, president of the Webb Ranch family corporation.

He said he was relieved to hear that the drivers' injuries were not severe.

"We're just trying to understand why it happened," Hubbard said. "The fact that (the horses) would go more than a mile, that's just bizarre."

It's the first accident like this at the ranch, Hubbard said. He said he's trying to find out why a paddock at the ranch was left unlocked, allowing the horses to wander.

"Somehow it didn't get locked up last evening," Hubbard said. "We don't know when they got out."

Portola Valley horse trainer John Charlebois, who responded to the crash, said he couldn't recall when that many horses had been killed in an roadway collision. "It's probably the worst horse-related accident in 20 years in Northern California," Charlebois said.

As investigators reconstructed the mishap, at some time Friday night or Saturday morning, four horses wandered from a paddock that had been left unchained and wandered up Portola Valley's Alpine road and onto the northbound lanes of 280.

At 4:50 a.m. a Toyota Prius driven by Richard Stein, 65, of Sacramento, plowed into three of them as they apparently tried to cross the freeway. The fourth horse, a mustang named Milo, stayed in a culvert beside the roadway and was unharmed.

The collision flung the three horses across the roadway. Traffic was blocked while their bodies were removed and the fourth missing horse was found and led to safety.

Battalion Chief Kevin Butler of the Woodside Fire Department said when he arrived one horse was still alive.

"It was raising its head and its front feet," he said. It died soon after.

While emergency crews set up flares and attempted to clear the roadway, a Mercedes driven by physician Jean Gillon ran over the body of one of the horses. The car shot into the air, landed and spun around.

Gillon, 61, of Los Altos Hills, reached at Stanford Medical Center where she was being treated for two fractured vertebrae, said she was on her way to an appointment at Sequoia Hospital when she saw flares and emergency lights in the darkness ahead.

Thinking she was being waved into an outer lane, she said she slowed and continued moving when she struck the downed horse.

"It felt to me like I went up in the air about 3 to 5 feet, she said. "The car landed and I pulled myself out, laid flat on ground and waited."

Dead are a quarter horse named Maverick and two thoroughbreds named Euro and Rowan, according to Hubbard, the Webb Ranch president.

Kevin Chambers, who runs the nearby Portola Valley Training Center, was called to the scene and was able to lead the fourth horse to safety.

"He was scared," said Chambers. "I was able calmly walk up to him and put a lead rope on his neck. The vets gave him little tranquilizer to calm him down so we could walk him off the freeway."

Chambers said the deaths of his neighbor's horses' saddened him. "Thankfully no person was severely injured, but it's a bad deal. This is our livelihood. This is what I do."

Contact Daniel M. Jimenez at djimenez@bayareanewsgroup.com. Follow him at Twitter.com/DMJreports. Contact Pete Carey at 408-920-5419. Follow him on Twitter.com/petecarey