The air-conditioned Bangkok-Nakhon Si Thammarat bus went into the ditch on the Asia Highway between Tha Chana and Chaiya districts at 4.52am, police said. 
The Sri Suthep Tour Co bus was carrying 36 passengers. 
Fifteen of the injured passengers were sent to Chaiya Hospital, from which one was transferred to Surat Thani provincial hospital in serious condition. 
The four others injured were treated at Tha Chana Hospital. 
A passenger who was unhurt said he was sitting behind the driver and noticed the bus was exceeding 100 kilometres per hour in thick fog.

Bus flips in Surat Thani, 19 passengers injured

Nineteen passengers were injured in Surat Thani on Monday morning when their bus, allegedly travelling at high speed in foggy conditions, plunged into a ditch between lanes and rolled.