Not trying to argue the 4500 death as that seems wrong according to your link. however, i never understood the focus on "killed" as that seems to be a combination of things, i.e. EMT, medical services, car safety for example. Why not measure the accident by impact (that would keep it consistent over time).
Per the same link all forms of injury have also dropped, from serious to minor. They’ve been dropping for decades, despite rising population, car ownership, and traffic rates (with the exception of 2010).
Also, what happened in 2010?