Wichit Police and rescue workers arrived at the scene to find a truck with a trailer, registered in Ranong province, wedged among multiple damaged cars and a detached truck bed.
The truck driver, named by police as Sopak Honsoda, told officers his brakes failed as he entered the tunnel, leaving him unable to stop before ploughing into the vehicles ahead.
Preliminary checks confirmed no alcohol or drug use, Wichit Police Superintendent Pol Col Somsak Thongkliang said.
Three people were taken to hospital with chest pain and other minor injuries. No fatalities were reported.
“Initial findings indicate reckless driving causing injury and property damage,” said Wichit Police Investigator Pol Lt Col Chonlathit Khiao-on, adding that formal charges will depend on medical reports.
Truck driver Sopak had admitted responsibility and police have contacted his employer and the truck’s insurer to arrange compensation for victims, Wichit Police reported.
Traffic in both directions was brought to a standstill while emergency teams cleared the wreckage. Police have urged motorists to take extra care when entering the underpass, especially when travelling downhill.
Wichit Police confirmed the underpass was clear for normal traffic to resume just after 3:30pm.