REVERE – It was a nightmare after Christmas for thousands of commuters Wednesday when a large truck clipped the Revere Beach Parkway bridge that crosses over Broadway/Route 107 causing thousands of pounds of granite and concrete to fall onto the ground below.The truck is used to transport and unload dumpsters and had its rails in the upright position when it started scraping the undercarriage of the overpass at about 4:20 p.m.”It scraped the underside of the bridge and continued on to hit the lip of the bridge ? causing a significant amount of granite blocks to fall,” said State Police Major Kevin Kelly.The boulders that covered Broadway dropped mostly from the outer side of the bridge that faces downtown Revere. The truck also took down a horizontal sign that read, “Clearance 13’6″.”The driver, Willy Hall, 52, of Lynn, works for Casella Waste Management of Peabody and says he drove the same truck underneath the overpass more times than he could count.Hall had just dropped off an empty container at a Chelsea elementary school and was heading back to Peabody when the accident happened.”Apparently he left the arm up on the truck,” Kelly said.Earlier, Hall said he had no idea the rail was in the upright position.”I wouldn’t have took off if I knew it was up,” Hall said. “I heard a noise and I didn’t know what was going on.”Hall says he’s worked for Casella Waste Management “for eight months, but probably not much longer.”It was not immediately clear if Hall was cited.A bridge inspector was called in to evaluate the damage and later determined the bridge is structurally sound. Officials said beams on the bridge were only installed about 25 years ago and many of the granite blocks that fell were cosmetic, all of which minimized damage to the bridge’s structural integrity.However, before the inspector ruled the bridge safe, police ordered both sides of Revere Beach Parkway (Route 16) and Broadway closed for hours n right in the middle of the evening rush.”We didn’t want any vibrations on the bridge to cause further damage,” Revere Deputy Fire Chief Michael Conley said.Route 16 eastbound was closed at Webster Avenue in Chelsea and westbound traffic was detoured at Wilson Street in Revere. The on-ramps from Route 1 to Route 16 were also shut down.Traffic was gridlocked in both communities as automobiles crawled to their destinations.By 6 p.m., officials re-opened all eastbound lanes of Route 16 and one lane on the westbound side.As the Item went to press at 10 p.m. Wednesday, Broadway was still closed at Route 16 in both directions.