Morgan Lake describes what it was like to fall off the Chesapeake Bay Bridge in her car, and what she did to survive. (The Washington Post)

After the shattered glass and shards of plastic fender have been swept away from accident scenes on the Chesapeake Bay Bridge, all that remain are black marks on concrete barriers that divide slender lanes of whizzing cars from a terrifying plunge into the water.

The latest additions — a new splash of white, a long dark smudge and a handful of curiously placed notches cut through the concrete crown — were scrutinized Monday by four accident investigators.

The automobile group AAA asked federal safety experts to get involved, addressing the bigger question of whether those barriers are tall enough to keep vehicles from falling off the bridge.

As investigators worked to reconstruct the most recent incident — a Friday night collision in which a ­tractor-trailer rammed a car so hard it ended up on the barrier, then toppled into the bay — Mary Grover Ehrgood rolled blissfully across the bridge. She was bound from her beach house to her home in the Chevy Chase section of the District and was aware that good fortune had narrowly saved a young female driver from a moment that lives in Ehrgood’s nightmares.

“My personal anxiety is about getting stuck on the bridge alone,” said Ehrgood, a real estate agent, “and it is for that reason that I want to have someone drive me over this bridge.”

Morgan Lake survived after her car plunged several feet from the Chesapeake Bay Bridge. She is photographed in Dunkirk, Md. on July 22. (Marvin Joseph/The Washington Post)

And so, she relaxed in the passenger seat of her BMW convertible Monday morning as it hummed along.

In the height of summer, more than a dozen people a day pay $25 to have someone drive them across the 4.3-mile-long twin spans, which arch 186 feet above the lower bay at its most narrow point. For many, the bridge is just another stretch of highway, albeit strung high above the water. For some, however, the lanes feel too narrow, the height too much to bear and the short concrete barriers scant protection if anything goes wrong.

They will pay the Kent Island Shuttle Service to drive their vehicles across. Some are like Ehrgood. Others turn their back toward the bay and fiddle with their cellphones. One daredevil speedboat racer cowers in the back seat with a blanket over his head.

“It’s a coping strategy,” said Debbie Mathews, a Kent Islander who has been driving people across the bridge for seven years. “He goes 130 miles an hour in that boat, but he hides in the back, and I tow his boat over the bridge.”

The two bridges have an unusual ability to make some drivers feel claustrophobic. Many other major bridges appear to have wider lanes or shoulders and higher bulwarks between the traffic and what lies below. The outer lanes on much of the two bay bridges — one opened in 1952 and the other 21 years later — are lined with what are commonly known as Jersey barriers, concrete walls three to four feet tall.

From the higher perch of an SUV or pickup truck, those barriers virtually disappear, creating an eerie fear for some that they could tumble over the side.

But Maryland Transportation Authority Police said that Morgan Lake’s red 2007 Chrysler Sebring did not leap over the barrier on its own or through any fault of the driver.

They said Lake had just cleared a toll booth and crossed onto the eastbound bridge at dusk Friday evening. That’s the older bridge. With the concrete barriers set within six inches of the two traffic lanes, there is little room for driver error.

The eastbound traffic at that time was heavy. A big red-cabbed tractor-trailer followed Lake up the slope of the bridge, moving faster than cars ahead of her. It struck her from behind.

After the truck made contact, the car rode up the wall, wound up on top of the wall and then rolled into the water. Sgt. Jonathan Green, a spokesman for the Maryland Transportation Authority Police, called it “a singular process,” in which the impact of the truck hurled the car into almost 10 feet of water.

It was less singular for Lake.

“I was dragging against the Jersey wall of the bridge, and I thought I would be okay if I could stay on the bridge,” Lake said. “As soon as he hit me, I flew, and then I flew up on the wall of the bridge.”

Glass exploded, and air bags popped. She screamed as the car fell 27 feet and hit the water. Her mouth and eyes were closed.

“I felt like I was drowning, and I didn’t want to,” she said. “I visualized my car and pushed the seat belt.”

A broken window was her exit.

“I pushed the side of the car with my feet to get some momentum to reach the surface.”

She swam to a rocky support piling for the bridge, where a boater came over and swam across with a life jacket. Marine police arrived in rescue boats next.

On Monday, AAA wrote to Deborah A.P. Hersman, chairman of the National Transportation Safety Board, asking her independent federal agency to determine whether the bridge’s railings are high enough to prevent such accidents.

“We believe this crash merits investigation by the NTSB, and call upon your agency to undertake an investigation,” AAA said in the letter. An NTSB spokesman said that the letter had been received but that the agency had no immediate response.

There’s no way of knowing whether Friday’s crash made more people fearful of the bridge crossing, but there is no shortage of people in the area who tremble as they approach.

“I have a linebacker, used to play for the Maryland Terps,” said Mathews, the shuttle service driver. “He’s got a Cotton Bowl ring. And he said his heart used to race so bad going across the bridge. He said, ‘By the time I got off, it took me an hour to get myself calmed down.’ ”

Men and women are equally represented among those who pay to be driven across the bridge, she said. They come in all ages.

The biggest recent panic attack came Saturday afternoon, when a woman headed east froze and refused to go any farther just after she drove onto the bridge. Police arrived to block the lane, Mathews was summoned to collect the woman and her vehicle was towed.

Better to invest in a driver than be overcome by anxiety and pay for a tow truck, said Ehrgood, the Chevy Chase real estate agent.

“There are a lot of people who really suffer,” she said, “and if you can afford the $25, it’s a gift to yourself.”