The accident occurred in the eastbound lanes of Route 422 near Township Line Road at 2:20 p.m.
State police said James Williams, 51, of Harmonyville, Pa., was stuck under the engine of the vehicle for approximately 20 minutes while emergency personnel worked frantically to rescue him.
According to PennDOT assistant press secretary Gene Blaum, PennDOT was sealing cracks along the roadway and had a traffic pattern set up to block the right travel lane.
"The operation was done and we were re-opening the lanes," said Blaum. "Williams was waving to the traffic with flags to stay to the left while the crew members were picking up orange cones."
Blaum said a PennDOT dump truck, driven by Paul Smith Jr., 46, of Birdsboro, Pa., was backing up in the work area at approximately five mph.
Smith was unable to see Williams and accidentally struck him, said Blaum. Williams came to rest directly under the engine section of the dump truck.
Upon being freed from the truck, Williams was secured and placed on a stretcher. He was rushed to a nearby Life Flight 3 helicopter and airlifted to Hahnemann Hospital in serious condition.
The incident is still being investigated.
"We're waiting for further information on this accident," said Blaum. "Our thoughts are with Jim, his family, and the members of his crew."
Blaum said Williams is a 30-year veteran of PennDOT's Montgomery County Maintenance Department at the Sanatoga Stockpile.
Assisting at the scene were Limerick Police Department; Linfield Fire Department; Limerick Fire Department; Humane Fire Company; Friendship Fire Company; and Royersford Ambulance.