Pol Lt Jakrit Sesnavej of Bang Pakong district police station said the accident occurred at 7.45am in front of the electric wire factory Hanava Electronics (Thailand) Co.
The woman was identified as Prasert Yulab, 42, a native of Muang district in Sukhothai province. She was driving her modified sidecar selling grilled pork and chicken along the highway.
Darika Nimking, 36, a resident of tambon Bang Samak and an employee of the factory, said while walking to the electric wire production plant she saw a wheel fall from a green trailer-truck heading for Chon Buri. The wheel flew over the expressway railing and dropped down onto the highway, hitting Mrs Prasert and then a power pole.
The truck kept heading towards Chon Buri, probably unaware of what happened.
Pol Lt Jakrit said it was an accident. The trailer-truck was registered in Bangkok with the licence plate 70-0631. Chon Buri police were contacted to intercept the vehicle, he said.