Thai drivers are able to follow the rules if the alternative is paying a lot of money.
I once was in a van from Hat Yai in Thailand to Malaysia. On the Thai road there was chaotic traffic. On the same road which continued in Malaysia, the traffic was very civilized. According to the driver the reason was easy. If they are stopped in Malaysia they have to pay a lot of money. So, they behave.
Police could enforce the rules in Thailand as well, if they would want to do that.