MOTOR VEHICLE SAFETY ACT (Thai Law)
Section 42
Anyone who wishes to drive a motor vehicle on public roads must possess an appropriate driver license.
The driver must carry the driver license and a photocopy of the registration book and show them to competent officers upon request.
This does not apply to those who are learning to drive a motor vehicle according to the provision of Section 57.
If the driver is an alien who doesn't have an immigrant visa, he may drive a motor vehicle with a driver licence specified in the Section 42-2.
In such a case, he must carry documents specified by the treaty between the Thai government and the government which issued such driver licence, and show them to competent officers upon request.
Section 42-2
In case there is a treaty between the Thai government and a foreign government regarding mutual acceptance of driver licence, an alien who does not have an immigrant visa may drive a motor vehicle with a driver licence issued by such a foreign government, or an automobile association authorised by such a foreign government.
Once you obtain a non-immigrant visa or establish you are a resident (such as enrolling kids in school, buy a car etc,) and are no longer a toursit, you need a Thai drivers licence as your national license and international driving permit is only legally accepted if you are a tourist. Several insurance companies have fine print stating that the driver should hold a valid Thai driving licence to be fully covered after a certain amount of time in Thailand.