Q. How can a police officer know if a driver is impaired by drugs? With alcohol there are roadside tests like walking a line and a breath test. Are there similar tests for drug impairment? Alcohol has a .08 limit, and now cannabis has a limit, but are there limits for other drugs?

A: It’s time for a pop quiz: At what point has a driver had too much alcohol to legally drive? This isn’t exactly a trick question, but if your answer was a blood alcohol content of .08, well, you’re wrong.

It’s true that the legal limit in Washington is .08 BAC. But a driver can be arrested for DUI below a .08 BAC. That is the BAC at which a driver could get a DUI with no additional evidence.

But imagine this scenario: A driver weaves over the fog line, and then drifts across the centerline into the oncoming lane. An officer pulls the driver over. When the officer approaches the vehicle, the driver rolls down the window just enough to hear the officer. (And we’re going with “he” because of statistics. Over 75% of impaired driving fatalities involve male drivers.) The driver refuses to do field sobriety tests or provide a breath sample, but the officer can smell alcohol coming out of the car, hears the driver’s slurred speech and watches as he fumbles to slide his driver’s license through the barely open window.

Does the officer have enough evidence to arrest the driver for DUI? The law says, in part, that people are guilty of impaired driving if they drive a vehicle “while the person is under the influence of or affected by intoxicating liquor, cannabis or any drug.” The driver in this example was clearly impaired. And even if the driver later decides to provide a breath sample and it comes in at .07 BAC, it’s still a DUI. The law doesn’t require that a driver reaches .08 to get a DUI, and plenty of people have been arrested with a lower alcohol level. The effects of alcohol begin long before reaching a .08 BAC.

Also, DUI includes impairment from other drugs, but recognizing it takes additional expertise. Alcohol is by far the most commonly used impairing substance (62% of adults drink alcohol; 19% use cannabis, the next most frequently used drug), so plenty of us have witnessed the effects of alcohol. But could you list the common indicators for narcotics or hallucinogens?

Many officers have had additional training to recognize the effects of various drugs, and some have become drug recognition experts. These officers are able to identify when a driver is impaired by a specific category of drug. That’s important; we don’t want people getting arrested for DUI because of diabetic shock or other medical emergency, even if some of the symptoms are similar to impairment. And we don’t want officers allowing an impaired driver to stay on the road because they don’t notice the indicators of impairment for less common drugs.

The real threshold for violating our DUI law is the moment a person drives while affected or impaired by alcohol or any other drug.

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