Where is the law saying former Presidents can't drive?

Factual Questions
21

If you are in prison, can you drive?

In the context of OP’s question, it’s a self-solving problem.

22

It was a new question surrounding “secret service protection” which is the foundation of the answer to the OP.

23

For more on the Secret Service’s control, or lack thereof, over a President: Is the Secret Service responsible for keeping the president from getting drunk? - The Straight Dope

24

Considering the news reports of drunken secret service agents, that’s a funny one.

Try:

https://www.google.com/search?q="secret+service"+drunk&rlz=1C9BKJA_enUS1055US1055&oq="secret+service"+drunk&aqs=chrome..69i57j0i512l2j0i7i30j0i390i650l4.19567j0j4&hl=en-US&sourceid=chrome-mobile&ie=UTF-8