Police said a boom truck was lifting the beams when crane operator Jeff Baker, 39, of Mount Savage, Md., lost control.
Timothy Whalen told police that he saw the beams swing into the roadway, but he was unable to avoid a collision.
One other vehicle was slightly damaged by debris that came off the truck, Maryland State Police Cpl. Phil Martin said at the scene.
The accident shut down eastbound Interstate 70 near the exit for Md. 66, which runs between Boonsboro and Smithsburg, state police said. The closure resulted in a backup of several miles along the interstate. Traffic on eastbound I-70 was backed up beyond Md. 65, and U.S. 40 traffic was extremely heavy, police said.
At 12:10 p.m., state police said both lanes of eastbound I-70 were reopened in the area of the crash.
Trooper Lee Cain said at the scene that a state police reconstruction team and Occupational Safety and Health Administration personnel were on the way to investigate the accident.
RNG Construction Inc. of Joppa, Md., was the company that was operating the crane when the accident occurred, Maryland State Highway Administration spokesman Charlie Gischlar said Wednesday afternoon.
He said sign installation near the accident scene has been suspended until an investigation determines what went wrong.
"We're looking into what happened here," said Gischlar, adding that RNG waited until rush hour was over to begin posting the signs.
RNG did not return a telephone message Wednesday seeking comment.
At the scene, two white sneakers lay on the ground, one on either side of the truck. The driver's seat was on the ground and the passenger's seat appeared to have collapsed. The dashboard panel was pushed toward the back of the cab.
Police said the Whalens were co-operators of the truck.
Fire companies from Funkstown, Mount Aetna and Myersville, Md., were on the scene along with two ambulances and a Washington County medic unit, a dispatcher said. By 10:15 a.m., some of the firefighters were leaving the scene.
Staff writer Julie E. Greene contributed to this story.