Apple very reasonably has decided using your phone while driving requires voice support. It's not about "traction."

People have used buttons in their cars for years. I’d be curious to know whether Siri is actually safer.

People have not been responding to text messages or browsing their entire music library in their cars for years. Siri also doesn't have to be safer than buttons, it just has to be safer than touching your phone for it to make sense as a UX requirement.

I don't believe people have used buttons in their cars specifically to control their phones, though.

I do agree that it would be nice to know whether voice control is safer, but unfortunately I can't find any studies that compare voice vs tactile control (all I can find is studies on touch vs tactile)

If the voice control was reliable it would absolutely be safer, it's like telling your passenger to change the AC, except they don't have to move and bother you.

If the voice control is bad and you are never sure if it heard it correctly then it wouldn't be safer.