Legal Question in Criminal Law in Washington

In Washington, that case, with no prior history, probably stays in juvenile court.

What you are describing is probably Vehicular Homicide; a 'violent' offense, but not a 'most serious' offense. So if there is not a prior, original jurisdiction stays in juvenile court. Strangely, if a 16 year old commits DUI by itself (any driving misdemeanor or gross misdemeanor), the local adult court gets it. In a felony situation like yours, juvenile probably keeps it.

Every case is different, and different counties handle cases differently. This is only generalized information.

RCW 13.04.030, Juvenile court�Exclusive original jurisdiction�Exceptions.

... [T]he juvenile courts in this state shall have exclusive original jurisdiction over all proceedings: ...

Relating to juveniles alleged or found to have committed offenses, traffic or civil infractions, or violations as provided in RCW 13.40.020 through 13.40.230, unless: ...

(v) The juvenile is sixteen or seventeen years old on the date the alleged offense is committed and the alleged offense is:

(A) A serious violent offense as defined in RCW 9.94A.030;

(B) A violent offense as defined in RCW 9.94A.030 and the juvenile has a criminal history consisting of: (I) One or more prior serious violent offenses; (II) two or more prior violent offenses; or (III) three or more of any combination of the following offenses: Any class A felony, any class B felony, vehicular assault, or manslaughter in the second degree, all of which must have been committed after the juvenile's thirteenth birthday and prosecuted separately;

RCW 9.94A.030(55) "Violent offense" means: ...

(xiv) Vehicular homicide, when proximately caused by the driving of any vehicle by any person while under the influence of intoxicating liquor or any drug as defined by RCW 46.61.502, or by the operation of any vehicle in a reckless manner;