Lowering the speed limit from 30mph to 20mph in built-up areas has been one of the most controversial law changes made by the Welsh government.
It was introduced to save lives and get more people walking and cycling, and so far has been enforced through engagement with the public since January.
But anyone breaking the 20mph limit could face points and fines from Monday on roads where engagement has not worked.
Ministers say the £34m law, which came into effect six months ago, will reduce deaths on roads.
But it has not proved popular with many, with hundreds of 20mph signs being defaced.