If you’re going with the flow of traffic on the Capital Beltway, Interstate 95, I-270 or I-66, there’s a reasonable chance that a driver near you is on the verge of falling asleep this week.

Nearly a third of people surveyed nationally said they’d almost fallen asleep in the past month, and studies have shown that the twice-annual time changes throw people’s body clocks into a sleepy tailspin for about a week.

“Asleep at the wheel” has become such a common metaphor that it’s a cliche, but in the literal sense it gets a lot less attention than drunken or distracted driving. It has been estimated that 1.9 million crashes a year, and one in six fatalities, involve a sleepy driver.

“Tired drivers pose a safety risk because fatigue can degrade every aspect of human performance,” said Deborah A.P. Hersman, chairman of the National Transportation Safety Board. “It slows reaction time, impairs judgment and degrades memory.”

Drowsy driving has been listed as a high-priority concern by the NTSB for more than two decades. The board has called on regulators and employers to develop guidelines and fatigue management systems.

“Drowsy driving has always played a significant role in causing crashes,” said Capt. Susan H. Culin, commander of the traffic division of the Fairfax County police. “You often pull over a person who appears to be driving under the influence and it turns out they’re just falling asleep.”

She said tired drivers often drift into an adjacent lane, roll through a red light or miss a turn because of a lapse in concentration.

The AAA Foundation for Traffic Safety produced a study last year that said 41 percent of drivers indicated they have fallen asleep or nodded off at least once. It said those who fall asleep are more likely to be younger, male and driving alone.

In a subsequent survey in June, the foundation reported that 32 percent of drivers said they had driven while “so sleepy that [they ] had a hard time keeping [their] eyes open” in the past month.

One of the most spectacular fatal crashes in recent years occurred in 2008 when a 19-year-old woman fell asleep on the Chesapeake Bay Bridge, drifting into the oncoming lane and causing a tractor-trailer to career off the bridge into the water below.

The issue has drawn particular attention in the aftermath of crashes involving long-haul truckers and intercity buses. They have resulted in closer regulation of the time drivers are allowed to stay behind the wheel and mandatory rest periods between shifts.

Fatigue has been singled out or suspected as a contributing factor in aviation and rail accidents. Investigators concluded that the pilot’s mistakes caused the 2009 crash of a Colgan Air plane that killed 50 people and that his performance was probably impaired because of fatigue.

Although drivers gained an hour of rest this weekend when clocks were set back after almost seven months of daylight saving time, the temporary disruption of established sleeping patterns causes fatigue.

“Studies show that traffic accidents noticeably increase for a week following the time change in both the fall and the spring,” said John B. Townsend II of AAA. “Motorists have a tendency to misjudge the impact being tired has on their driving ability. That puts themselves and others at risk.”

He said the foundation’s research pointed to one particularly troubling finding.

“What’s so alarming is that over half of these drivers reported having fallen asleep while driving on high-speed roads,” Townsend said. “Drowsy driving kills.”