The pickup truck was carrying 24 passengers, who were construction workers, in the back when it skidded off the road and hit a roadside tree.
The victims comprised of 10 men, including the driver and 6 women, were killed. The rest were injured and taken for hospital treatment.
"The accident happened on a straight stretch of the road and there was no rain. No other cars were involved. At this point, it's likely he was falling asleep while the car was traveling at high speed," Pol Col Suthee Setthawong told the Associated Press in a telephone interview.
Police also found a tablet of methamphetamine in the driver's pocket and a bottle of alcohol next to his seat, Suthee said.
Police said they were working to identify the construction workers, as well as their nationalities.