Speed limits are posted all over rural Thailand.
However, with no police patrolling the roads, drivers are either driving super fast or turtle slow.
Double truck with trailer is 60 km/hr
Bus or regular truck is 80 km/hr
Cars and Motorbikes are 90 km/hr
In rural Isaan the super fast drivers will often pass on blind corners and other unsafe driving behaviors.
Never expect them to be professional drivers and you should always yield first to avoid any collisions.