Along the Quy Nhon-Chi Thanh and Chi Thanh-Van Phong sections through Dak Lak Province, cones and obstacle warning signs were placed at emergency stopping areas for drivers to use when their vehicles break down.
The devices warn approaching motorists to slow down or change lanes, but hundreds have disappeared, with some replacements stolen two or three times.
Driver Nguyen Van Thang said the equipment was widely available when the expressway first opened, but its disappearance from many locations had increased risks for vehicles forced to make emergency stops.
Thefts have also been reported on the Quang Ngai-Hoai Nhon, Hoai Nhon-Quy Nhon and Van Phong-Nha Trang expressways.
Road Management Zone III, which oversees five expressway sections between Quang Ngai and Nha Trang, said more than 500 traffic cones and 70 warning signs had been stolen. Although replacements are regularly installed, some locations remain without equipment because of the scale of the thefts.
On the Chi Thanh-Van Phong Expressway alone, 100 cones and 30 warning signs at 50 locations were stolen within about three months of the road opening, according to Pham Truong Vu, executive director of the project.
The project investor plans to replace the missing equipment and complete a surveillance camera system linked to the traffic management centre.
Authorities said traffic violations and theft or damage of state property would be handled in accordance with the law.