I don't think it's about health,if it's was they'd ban cars themselves

Is a car deemed a "public place"?
The Interior Of Your Car Is A Public Place | JONATHAN TURLEY
And being enclosed, deemed a 'enclosed public space'?
If that's the case it's been illegal to smoke in cars (regardless of age) since they restricted smoking in 'enclosed public places'.I dont recall seeing much of that being enforced.

Enforcing this silliness will only take away from more serious traffic issues.
Maybe advice over enforcement is the answer.
It's obvious you don't smoke in a car with a child,my uncle back in the day is about the only person I've seen do it
