Q: I know it’s not legal to drive with an open alcohol container in the car, but is it legal for a driver to drink alcohol in a parked car? Assuming the driver doesn’t drink enough to be impaired, have they broken the law?

A: Before I tackle the question about the law, I’d like to clear something up. There isn’t an OK amount of alcohol in your system when driving. Washington’s per se law sets the blood alcohol concentration at .08, but a driver can be affected by alcohol at well below that limit. And impairment begins earlier than you might think.

At a BAC of .02 (one drink or less for most of us) a driver experiences a decline in visual function and the ability to do two (or more) tasks at once, like maintain a safe following distance while checking mirrors and making a lane change, for example. By .05 drivers experience a reduction in their coordination, ability to track moving objects and response to emergency driving situations; their risk of a fatal crash increases by 40%. And I’m using conservative data; some studies show a 100% increase in fatal crash risk.

Getting back to the question, Washington’s open container law is pretty broad. It states that it’s a violation to drink or be in possession of an open container of alcohol “when the vehicle is upon a highway.” That’s more than driving on the road. The law defines a highway as, “the entire width between the boundary lines of every way publicly maintained when any part thereof is open to the use of the public for purposes of vehicular travel.”

If you can drive on it and it’s paid for with tax dollars, it’s probably a highway. And the boundary line isn’t where the pavement ends (that’s where the roadway ends); it includes the shoulder, any parking, and even the green space along the side of the road. Drinking a beer in a parking stall at a rest stop would be a violation, for example.

Even part of your driveway might be within the highway boundary. In many neighborhoods, the city right-of-way is much wider than the paved roadway. Sometimes that can result in homes where nearly all of their driveway is part of the highway.

Even if a parking lot is privately owned it’s probably a public place, and the state has a law that prohibits drinking liquor in a public place. The law has a long list of what it considers a public place, but you can sum it up as any place that is accessible and generally used by the public. Even though you have an increased expectation of privacy in your vehicle, if you do an action that’s visible to the public, like drink in your car, it’s a violation of the law. I wasn’t really sure about that so I described that scenario to a prosecutor who said, “I would prosecute that.”

The law does have an exception for the living quarters of a motor home, but there’s a requirement that you’re in the home part of the RV, not the motor part.

There’s a good reason why the law takes drinking in vehicles so seriously. Even though only a small percentage of drivers ever drive impaired, about half of all traffic fatalities in Washington involve impairment. Alcohol and vehicles are not a good mix.

Doug Dahl writes a weekly column for this newspaper. He is with the state Traffic Safety Commission.