Pol Lt Col Sophas Thanonthip, an inspector from Muang Samut Sakhon police station, said the accident happened at 11pm in front of a petrol station in Muang district.
Sophas said a crane being used to build an elevated expressway tilted and fell, totally blocking the outbound lane.
Construction workers took about half an hour remove the crane, allowing traffic to resume.
Sophas said a bolt that pinned the crane to its tractor base sheared off, causing the crane to teeter before crashing down on the road, flipping its truck-base.
Motorists managed to brake to a halt before hitting the obstacle and no injuries were reported, he added.
Rama II is notorious as a road of “never-ending” construction work that has been blamed for multiple accidents.
In August last year, a 3-metre-long 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.