Pivotal moment: Kennedy's assassination changed the way the Secret Service approached security. "For the most part, the Secret Service's preference would be to drive the former presidents at all times," said Mickey Nelson, who retired in 2012 as the Secret Service assistant director of protective operations after 29 years in the service. In a country as car-oriented as the United States, the draw of the driver's seat is strong. The security imperative is stronger. The threat to top US officials is ever-present, and the agents who squire current and former presidents are trained in evasive and defensive driving manoeuvres. Presidents still find ways to get their hands on the wheel - even if it's just a golf cart on the links or an off-road excursion on private property - often to the consternation of the agents assigned to protect them. The late President Ronald Reagan, restricted from driving on public roads, enjoyed a pair of older four-wheel drive Jeeps on his 688-acre Rancho del Cielo near Santa Barbara, California, his home away from the White House.

"He'd be in the driver's seat and I'd be in the passenger seat and I'd have a death grip," former Secret Service agent Stephen Colo said. "Everything was completely open. No seat belts," Colo recalled. "He's driving and I'm thinking: If we go over the edge, how I'm going to drag him out of that car?" President George W. Bush has a spread in Crawford, Texas, twice as big as Reagan's and, until this year, had a pick-up truck to match. He "drove his truck pretty regularly at the ranch," Nelson said. "He even on occasion drove other world leaders on the ranch in his truck." Bush, 67, whose main residence now is in Dallas, auctioned the 2009 Ford F-150 King Ranch 4x4 SuperCrew for charity for $300,000 in January, saying in a statement that he hadn't driven on a public road for "many, many years."

On official business both in Washington and away, US presidents ride in an armored Cadillac limousine known as "the Beast." Over the past five decades the size of the presidential vehicle and the blocks-long motorcade in which it rides has grown. "The critical moment was when JFK was assassinated," said H.W. Brands, a University of Texas professor and presidential historian. President John F. Kennedy's death by a sniper's bullet while riding in an open-topped Lincoln on November 22, 1963, in Dallas was a watershed for presidential isolation. Handing over the car keys is now part of the bubble that includes personal chefs, constant security, and aides to handle every mundane chore. Each succeeding president is further removed from the lives of ordinary citizens. Obama had to fight even to keep his Blackberry. "Once you become president, you don't even have to stop for red lights," Brands said. "And if it looks like traffic's too bad, you just take a helicopter."

He said Lyndon Johnson may have been the last president to drive on public roads after leaving office, saying he may have cruised around Texas Hill Country in addition to driving on his private ranch. Not owning a ranch or other vacation property, President Barack Obama, 52, gets his driving fix in other ways. He caused a stir in March when he was photographed driving a golf cart solo at the Andrews Air Force Base course. Usually if a president is behind the wheel of anything, there's a Secret Service agent alongside. The press pool report for the day called it a "unique presidential sighting". Obama drove something more powerful than a golf cart as recently as 2012 when he tooled around the White House grounds in a plug-in-hybrid Chevrolet Volt, a vehicle he had driven once before - for about 3 metres - in a GM plant two years earlier. "That was my big joy ride. Three times around the South Lawn driveway," Obama said on NBC's "The Tonight Show."

Biden, 71, hasn't given up completely on the idea that he'll one day drive again. He owns a 1967 Chevrolet Corvette, a green convertible with a 327-cubic-inch engine and side exhaust pipes. He told "Car and Driver" magazine in 2011 that being banned from driving is "the one thing I hate about this job." Bill and Hillary Clinton have lived much of their adult lives in the bubble. Bill Clinton, 67, was governor of Arkansas from 1979-1981 and again from 1983 until he won the presidency in 1992, and he then served in the Oval Office until January 2001. Mike McCurry, Bill Clinton's former press secretary, recalls one time the president took the wheel, and it didn't make the Secret Service happy. Clinton was visiting the late King Hussein at his compound in Jordan when the monarch wanted to show off his new high-end Mercedes. First, the king invited Clinton to ride along in the passenger seat, McCurry said. The Secret Service grew concerned when Hussein said "Bill, you drive." To the relief of the agents, the $150,000 car had a manual transmission, and Clinton didn't know how to use it, McCurry said.

Hillary Clinton, 66, later acquired a security detail in her own right when she was a 2008 presidential candidate and then as secretary of state, extending the time she's spent in the back seat rather than behind the wheel. "One of the regrets I have about public life is that I can't drive anymore," Hillary Clinton told the National Automobile Dealers Association in January. Bill Clinton has shared the same sentiment. "That's why whenever I'm on the golf course, I always make them let me drive the golf cart," he said in a 2012 appearance on "The Ellen DeGeneres Show." Bloomberg