SAN FRANCISCO » Shortly after President Barack Obama landed here one fall day for Democratic Party fundraisers, his motorcade pulled out of the airport and raced at 80 mph down an empty freeway to his hotel in the city.

At the front of the procession were bulletproof, black SUVs and limousines driven by Secret Service agents who had spent hundreds of hours learning how to maneuver at high speeds.

Bringing up the rear were police cars with their lights flashing and a Secret Service ambulance that follows the president wherever he travels.

And in between were several vans filled with White House staff members and journalists, being piloted by volunteers like Natalie Tyson, a 24-year-old Bay Area graduate student wearing fluorescent orange sunglasses.

"Wow," she exclaimed as she hit the gas and the van lurched within a few feet of the one in front of it. Then she slammed on the brake. Then she hit the gas again.

"Sorry about that," she said.

She returned her hands to the textbook 2-and-10 positions on the steering wheel.

Volunteers with no special training are a link in the middle of the fastest, and most high-profile, chain of vehicles in the country. They are cheaper than the Secret Service personnel or local police officers who surround them on the road. And their cargo of lowly staff members and reporters is apparently less precious.

The White House declined to comment on the practice. The Secret Service defended it, saying it had been standard since at least the 1980s. Volunteer drivers "are briefed by the Secret Service agent responsible for the motorcade prior to any movements" about what to do in case of an emergency, like an attack, a spokesman for the agency said.

But Tyson said in a telephone interview several weeks after she drove in the motorcade that she had received little instruction from the Secret Service about what to do in the event of a high-speed emergency. She assumed she should just follow the car in front of her no matter what happened.

"Whatever I am," she said, "is good enough for them."

"Good enough for them" is apparently having a driver’s license and a clean criminal record and knowing someone at the White House.

A week before Obama arrived in San Francisco, a childhood friend of Tyson’s from Cupertino, California, who now works at the White House, reached out to her to see whether she was interested in driving.

"He just texted me and said, ‘Do you want to volunteer as part of this and drive in the motorcade?’" Tyson said. "It was kind of sudden. I didn’t even know the president was going to be in town."

Tyson said her driving record was "pristine" and that she had "driven a pickup truck but not a van."

Some security experts said the practice was troubling. Not only could the volunteers cause an accident – and they have – but they are also sandwiched between the president’s limousine and the Secret Service ambulance, so neophyte drivers could create complications and delays in an emergency.

Dan Emmett, a Secret Service agent from 1983 to 2004 and the author of "Within Arm’s Length: A Secret Service Agent’s Definitive Inside Account of Protecting the President," said he considered volunteer drivers like Tyson, who read her family therapy textbook between stops, a national security threat.

"You are face to face with a young person who is just completely full of themselves and enthralled," Emmett said, recalling the years when he was part of the motorcade’s counterassault team that traveled in vehicles in front of the volunteers.

He added, "We were more concerned with that than an attack on the motorcade."

The spokesman for the Secret Service, Ed Donovan, pointed out that the motorcade does not move in normal traffic conditions, saying there is typically "no other traffic on the road at the time the presidential motorcade is moving."

The so-called secure package of vehicles that includes the president’s limousine and the agents traveling with him could easily detach from the vans and take off, Donovan said.

"As far as the ambulance being at the back of the motorcade, the doctor is in the secure package in the event they are needed," he said.

Privately, Secret Service officials said they did not use agents or uniformed personnel to drive the vans because it was not the agency’s responsibility to protect the White House staff members or journalists. White House officials said they were forced to use volunteers because staff members needed to be with the president at all times, and reporters demand that they travel with the president wherever he goes.

Often, the White House reaches out to campaign volunteers or friends to be drivers. One woman who drove a van during Obama’s trip to Arkansas in May said a mass email had gone out to students at the Clinton School of Public Service at the University of Arkansas in Little Rock asking whether anyone wanted to drive.

There is a long history of accidents in presidential motorcades dating back more than a century.

In 1902, a trolley car in Lenox, Massachusetts, struck and killed a Secret Service agent in President Theodore Roosevelt’s protective detail.

During the 2008 Democratic primaries, a motorcycle police officer escorting Hillary Rodham Clinton was killed in Dallas after performing a "leapfrog" maneuver to speed from one intersection to the next, sometimes done at over 100 mph.

While the drivers are not paid, the gig has its perks.

Shortly before Obama returned to Washington from San Francisco, White House staff members ushered Tyson and four other volunteers into a room at the InterContinental hotel for a photograph with him.

He shook their hands and told them they were the best drivers in San Francisco.

It was a common refrain for the president: Whatever city Obama is visiting, he assures the drivers that they are the best in that city.

"His hands," Tyson said, "are very soft."

"It was such a short visit. I had the idea I was going to say something interesting, like, to show him how funny and fun I am. But I just said, ‘Nice to meet you,’ which in hindsight was the right choice."

Chauffeuring for the president’s motorcade makes for the ultimate Facebook fodder. Tyson’s page is now filled with photographs of her adventure.

A photograph of her in the van was captioned, "Me, wondering why anyone would trust me with a 15-passenger van full of reporters."

Tyson was even able to get a photograph of the president’s limousine – a memento that got a security guard at the Centers for Disease Control and Prevention fired this year.

One photograph on Tyson’s Facebook page showed her standing in front of the president’s limousine, which is known as the Beast, flanked by two Secret Service agents. It received "so many likes you wouldn’t believe it," she said.

Tyson said her stint as a driver will be the ultimate icebreaker.

"You know how sometimes you are starting a new job or starting a new course, and you have to come up with three truths and a lie about yourself?" she said. "This will be my truth that sounds like a lie."

For the former Secret Service agent, Emmett, it is still a troubling practice.

"If the motorcade ever comes under fire, it’s going to be a problem," he said. "There are so many non-law-enforcement vehicles that it’s going to be a goat rope. Everyone will be responding, police officers and the Secret Service, and it will be all these people running around in a panic like the last scene of the ‘Blues Brothers’ movie, when there’s the big police chase that ends in a wreck of 50 police cars."

© 2014 The New York Times Company