Two people were killed and one was injured when part of a U-turn flyover fell on to Rama II Highway in Samut Sakhon on Sunday night.
The huge concrete beam, about 10 metres long weighing about five tonnes, together with its concrete rail guard fell onto three vehicles passing underneath the U-turn flyover that was closed for maintenance.
The beam hit a Chevrolet car, a pickup truck and an empty oil tank truck on the Bangkok-bound side of the highway.
A woman who was the front passenger of the car was killed instantly. Its driver was pulled out of the car by eyewitnesses. Five people on the pickup truck escaped injury as only the front of the truck was hit. Two workers were injured and one of them later died. They were working on the flyover.
The incident caused heavy traffic congestion on the highway as workers were removing the debris from its express lanes.
The Highways Department had closed the old U-turn flyover since June 23 for maintenance.
It had been heavily used by big trucks. The maintenance was earlier set to finish at the end of this month.