Lt Col Pongpipat Khamchompoo of the Thalang Police was called to the scene, the U-turn near Thai Watsadu on Thepkrasattri Rd in Moo 3, Srisoonthorn, at about 1pm.
Officers and rescue workers arrived to find a black Honda City sedan, registered in Songkhla, stopped in the middle lane of the road. The car had sustained damage to its front left bumper and its windshield was cracked.
The driver, Chuwit Sae-ng, 23, was at the scene waiting for police.
Just metres away was a grey Honda Wave 125 motorcycle, registered in Prachinburi, lying in the middle of the road.
Its driver, Suda Nooboonmee, 66, was still sitting on the road injured. She had suffered minor injuries from the collision
However, her passenger, Wanwisa Pathumwan, also lying in the middle of the road, had suffered serious head injuries.
An ambulance transported both women to Thalang Hospital.
Ms Wanwisa was deemed to be in critical condition and was transferred to Vachira Phuket Hospital in Phuket Town.
Col Pongpipat noted that from CCTV footage that had recorded the accident it could be seen that Ms Suda had rounded the U-turn and was moving slowly across the lanes.
However, she moved back into the right lane as a bus was coming through in the middle lane. The Honda car being driven by Mr Chuwit, heading northbound, then struck the motorbike from behind.
Col Pongpipat reported that officers were continuing their investigation and had yet to charge any person involved for the accident.