The accident occurred when the driver of the 18-wheel trailer truck heading downhill lost control and its trailer swept away five vehicles that were heading uphill from the opposite direction, said Pol Lt Col Anukan Thamwijan, an inspector from the Wang Khon Daeng police station.
The crash happened around 6pm between kilometre markers 45 and 46 in Buphram sub-district of Na Di district between Thap Lan National Park and Khao Yai National Park. It was raining heavily at the time.
The vehicles swept off the highway were two passenger cars, two passenger vans and a big motorcycle. All were registered in Bangkok.
Three people were killed instantly and one succumbed to injuries on the way to Na Di Hospital. Most of the 15 injured people were passengers in the vans. They were treated at Na Di and Kabin Buri hospitals, said Pol Lt Col Anukan.