While RCW 46.55.360 requires a vehicle to be impounded if the driver is arrested for DUI or Physical Control, there is no law that requires the police to a book a DUI suspect in jail. So the officer has discretion to either book a person arrested for DUI, or release them (call a cab, dropped off at a gas station, ect.) Typically, if the person has a prior DUI arrest, the chances of you being booked in jail increases. If you are arrested in rural King County, your chances of being booked in jail reduces since it will take the officer longer to transport you to the King County Jail in downtown Seattle.
If a person is booked in jail, a prosecutor must file charges against the person within 72 hours per CrRLJ 3.2.1(f)(1) or they will be released from jail. In King County, prosecutors very rarely charge a person with DUI right away. Instead, after being booked in jail the person will be released either right after posting bail, after they appear in front of a judge who releases them on your personal recognizance, or after the 72 hours have eclipsed since they were were arrested (whichever comes first). After the person is released from jail, the prosecutor has up to two years to file charges (for gross misdemeanors, like DUI). Unless the person spends a full 24 hours in jail, they may not get credit for the time they spent in jail towards thier sentence. So while posting bail will typically get the person released from custody more quickly, they may want to wait until they have served more than 24 hours in custody to insure they get credit for the time they spent in jail.
After the person is released from jail, it is important to pick up the vehicle from the impound lot as soon as possible. Impound fees are assessed on a daily basis. So the sooner the person gets their vehicle, the less the person will have to pay.
Go back to the initial stop and arrest.