An interaction of public policy and technology has reduced (and continues to reduce) deaths per million vehicle miles traveled.

One other thing to note here is that it isn't clear how much of the technology improvement is technological improvement in medicine. In particular, there's an argument that murder rates have gone down because people who would have died from from some injuries are now being saved. (See e.g. this summary. ) If so, this has likely also contributed to the reduction in automobile fatalities, although I'm not aware of any studies which have specifically looked at that impact.