I'll start with the obvious: remember that regardless of whether it may now, temporarily be OK to purchase drinks to-go from restaurants, it's never safe to drink and drive. It's dangerous, often fatal, and we at Schauermann Thayer see way too much wreckage in people's lives because of it.
With that warning…. Most people probably have a pretty good idea of what complies with or violates Washington's “open container” law, at the extremes at least. Have a six pack or growler of beer in the trunk of the car? You're good. Glass of Brandy in the cupholder? Not OK. But between those extremes, what does the law really allow, especially in the current COVID-19 situation where restaurants may have more leeway to sell drinks to go?
Specifically, as of early May Washington state allows restaurants with liquor licenses to serve pre-mixed cocktails to-go while restrictions on in-house dining continue. The rules are laid out in a May 6, 2020 Liquor Control Board bulletin. In order to sell such drinks, they must be purchased with a meal, and packaged in a container with a secure lid and in a manner designed to prevent consumption without removal of the lid or cap (think mason jar). Drinks having lids with holes (think coffee cups, straw holes) are not permissible for sale, nor are Styrofoam and plastic cups, even if sealed with tape or other materials.
As the situation relates to “open container” laws for the driver and occupants of the vehicle, the rules promulgated by the liquor control board say that the open container laws (found at RCW 46.61.510) still apply. These drinks may not be consumed in the vehicle, or transported in any place but the trunk. When there is no trunk, the drinks must be kept in another area of the vehicle not normally occupied by the driver or passengers (this does not include a utility or glove compartment, but may include underneath a seat outside the driver's reach).
The rules allowing sale of cocktails to-go are set to expire within 30 days of whenever the county where the seller is located enters Phase 4 of Washington's Safe Start plan.