The board says confusing highway signs along Interstate highway 75 in Atlanta were a primary cause of the accident.

The board also cited driver error and a lack of safety features such as seat belts.

Investigators said the bus driver thought he was still in an HOV lane when he drove onto an elevated exit ramp, plowed through a stop sign and hurtled from an overpass back onto the interstate below.

The crash killed five members of Ohio's Bluffton University baseball team, along with the driver and his wife.

NTSB investigators say Georgia officials changed the design of the exit signs after having trouble mounting them. They say the change deviated from federal guidance on pairing some exit signs together, but it did not amount to a violation of federal regulations.

No fatalities were reported at the accident site in the ten years before HOV lanes were introduced in the mid-1990s. The board said there have been three fatal wrecks in the decade since then.

Georgia DOT had no immediate comment. The agency has previously said it received no complaints about the interchange, although police reports show at least three drivers that had earlier wrecks at the site said they misread signs and did not realize they had left the HOV lane before they crashed.