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)