Murder, or attempted murder, is an intent crime in most states in the US. Generally speaking, that means that in order to be convicted, it must be proven in court that the defendant had the specific intent to cause the death of another person when they undertook the act that caused the death. That's why most traffic-related deaths are charged as manslaughter, or under traffic-specific statutes such as vehicular homicide or DUI resulting in death (depending on the specific state and circumstances).