It was the worst accident at the I-75 ramp at Northside Drive. But it wasn't the first.

At least 82 other drivers have crashed at a ramp on an overpass from which a bus fell Friday morning, killing four college baseball players, the bus driver and his wife.

Forty-eight of the wrecks happened at the top of the ramp, where it runs into Northside Drive, according to an Atlanta Journal-Constitution analysis of nine years of Georgia Department of Transportation data. Two people died in the previous crashes. DOT could not say how the number of wrecks at the Northside Drive ramp compared to other ramps.

National Transportation Safety Board investigators were at the bus crash site Saturday. They also examined the bus, being stored at Hartsfield-Jackson International Airport, in search of clues.

One thing is clear, said NTSB's Kitty Higgins: "The driver did not intend to exit on Northside Drive."

The federal agency plans to examine the ramp's configuration and the signs leading up to it.

As part of its investigation, Higgins said, the board will look at HOV and exit ramp designs nationwide — particularly at left-hand HOV exits. She called the design "rare."

There are six left-hand HOV exit ramps in metro Atlanta.

Higgins, in Atlanta from Washington for the investigation, said transportation safety board officials "haven't investigated an accident like this before" involving a left-hand HOV lane exit.

"I didn't see speed limit signs at the [Northside Drive] exit ramp," Higgins said. "It's an issue people in this community are aware of."

The left-hand ramp and its access may be a problem, said Fred Hanscom, director of the Transportation Research Corp., based near Washington. Hanscom serves on a committee that writes the Federal Highway Administration manual on traffic control — a guide that includes proper ways to give traffic flow information to drivers. His firm researches transportation engineering and highway safety.

"When highway engineers design traffic control devices, a major thing to consider is a concept called driver expectancy," Hanscom said. "When we design signs, we have to know how drivers behave.

"A major problem is that this [the Northside Drive HOV ramp] was a left-hand exit," said Hanscom, who noted he has not studied this particular ramp. "Most off-ramps in the United States are right-hand exits. Left-hand exits violate driver expectancy."

That doesn't mean they should not be used, he said, it means traffic engineers have to prepare drivers for the upcoming left-hand exit. If a sign is only at the exit itself, Hanscom said, "that's too late."

"There are a lot of principles of highway signing that may be violated here [approaching the Northside Drive ramp]."

Strikingly similar accident

Six years ago this week, a 75-year-old woman followed an identical path up the exit, crashing at the top of the ramp and coming to rest on Northside Drive shortly before 10 p.m.

The driver's sister, a passenger in the car, was killed. Police said the driver apparently fell asleep at the wheel.

In 1991, before the ramp was used for an HOV lane, a drunken driver got on it going the wrong way. He drove 10 miles on I-75 going northbound in southbound lanes, causing numerous wrecks and the death of a Marietta father of three.

So why do road designers design left-hand ramps, which have such potential for confusion? They are a way to add more ramps to a road without taking up much more space.

Road designers' "main constraint is they don't have enough right-of-way to put it anywhere else," Hanscom said.

Those who died in Friday's crash were identified by Bluffton University as sophomores David Betts and Tyler Williams, freshmen Scott Harmon and Cody Holp and bus driver Jerome Niemeyer and his wife, Jean, all from Ohio. Twenty-nine other passengers survived. The bus was carrying the Bluffton Beavers baseball team from Ohio to Florida, where they were scheduled to play in a tournament this week.

Sharon Bledsoe, a Chattanoogan who used to live in Atlanta, said when she heard about the bus crash, she knew the location right away.

"I knew exactly which [ramp] they were talking about," said Bledsoe, who recalled driving a minivan carrying her four children earlier this year when she had a scare. She realized just in time that veering left did not mean staying in the HOV lane, and swerved out of it.

"It's confusing," she said. Once a driver realizes the mistake, "there's not a lot of time for you to reduce your speed."

State Department of Transportation Director of Operations Stephen Henry said the configuration and the signs at the I-75 HOV off-ramp to Northside Drive are in line with federal safety standards, and in some cases exceed them.

He noted that the bus apparently did not slow down despite two signs warning a stop sign was coming, followed by the stop sign at the top of the ramp.

Police said they did not find skid marks, indicating the driver did not attempt to stop or there was a mechanical failure.

Henry said the difference between the HOV lane and the exit to the left is clear because the exit ramp begins with a lane dotted off from the HOV lane. He said the suddenness of the exit lane and arrows directing traffic left is not a problem, since the HOV lane continues.

"You have a dedicated lane," he said, describing the path intended for HOV drivers. "You never lose this double line" marking the right margin of the HOV lane.

Within specifications

In the two miles before southbound HOV drivers on I-75 arrive at the Northside Drive exit, they see more than a dozen road signs. But only when the driver has rounded a blind curve and come upon the Northside Drive exit is there any indication that keeping left means exiting the HOV lane, and the highway.

An off-ramp a little farther north presents another challenge for HOV drivers: To exit, they have to leave the HOV lane, cross every lane of traffic and exit to the right with the rest of the cars. They have a quarter-mile of warning to make the move to the right.

Henry said the I-75 HOV lane exits follow transportation safety rules, noting the Northside Drive HOV ramp abides by or is better than standards advised by the Federal Highway Administration.

"It's clear you're not on the main line," he said. "The striping is well lit. There are exit signs. There is barrier wall on both sides."

Asked if the bus driver might have been confused all the same, he said, "These are professional drivers."

On Saturday, DOT spokesman David Spear said the department was "very anxiously" awaiting NTSB's findings.

"Whatever the ultimate cause ends up having been, six people lost their lives, and we certainly want to make sure that our signage, our configurations — while they meet standards — if there's a way we can improve them we want to try and do that," he said.

Computer Assisted Reporting Specialist Megan Clarke and staff writer Rosalind Bentley contributed to this article.