1. He is driving in the road.

Question

2. He is driving on the road.
3. He is driving at the road.
[Is #2 correct? What is the difference?]

4. He is playing at the playground.
5. He is playing in the playground.
[What is the difference between 'at' and 'in'? Are both okay?]

Answer 1

I'd use 2 and 4 -- not the others.

It's hard to explain customary usage -- rather like phrasal verbs and idioms. They are the normal (customary) way to express these ideas, that's all.