The trailer truck belongs to Export Transport Co and was delivering steel structure to Karnchang Plc, a contractor for the railway. It hit the flyover on a bypass road in Nonthaburi province around 9.15pm.
A concrete section of the flyover then dropped on top of two empty vehicles parked on the side of the street. A Nissan was wrecked and a Mitsubishi suffered damage to its front section.
The road was closed to all traffic for the rest of the night while the concrete debris was removed. It reopened to traffic about 7am Friday, according to the Facebook page of traffic radio FM. 91 Trafficpro.
The flyover had to be cut into three sections before it could be removed from the street.
The truck driver fled the scene of the accident, police said.