The accident happened at 12.33pm on the highway’s kilometre 16 post, opposite Phan Thai Norasing Shrine, highway police said.
The trailer truck hit the frame of a container set up to hold cement that would be poured to make a concrete beam.
The metal frame fell on the truck, immobilising it.
The truck’s rear then blocked a lane of inbound traffic, backing traffic up for more than four kilometres before construction workers were able to free the truck about an hour after the accident.
Police simply said the accident happened at a construction site under the elevated expressway being built there. Photos from the scene show the accident happened at one of several pillars being built to support a U-turn bridge.
The truck driver was sent to Ekkachai Hospital but did not suffer severe injuries.
He told police that another trailer truck cut him off, forcing him to swerve left.
Rama II is notorious as a “never-ending” construction site where multiple accidents have occurred.
In August last year, a three-metre metal bracket fell onto Rama II from a truck at an overpass construction site on Bang Khun Thian-Ekachai highway.
A driver passing beneath narrowly escaped death when his car was hit by the bracket.