EVERETT — With marijuana stores soon to open across Washington, state traffic safety leaders plan to gather survey data in Snohomish County this weekend to find out how many impaired drivers are on the state’s roads.

They’ll do another survey later this year for a comparison.

They worry that gains made in reducing drunken driving over the years could be undermined with increased drug-impaired driving.

The goal of the survey is to find out how many people are driving with drugs, alcohol, or both in their system. The survey will look for 75 legal, illegal and over-the-counter drugs.

It is part of a $1.25 million study, with the National Highway Traffic Safety Administration picking up the tab.

The work will be done by the Pacific Institute for Research and Evaluation, which has been doing national studies on impaired driving since 1973.

“This is really unique in the sense it’s a state view, not a national view,” said Jonna VanDyk, who is managing the study for the Washington Traffic Safety Commission.

Researchers are surveying drivers from six counties across the state in June. They were in Yakima and Spokane last week.

In Snohomish County, they’ll go to five different spots over the weekend. They aren’t saying where those locations are and don’t want people showing up to volunteer because they want a randomly selected sample.

The surveys will be voluntary and anonymous. The goal is 150 surveys in Snohomish County.

Drivers who take the survey will be asked about their drinking, drug use and driving behaviors. They also will be asked if they are willing to give a saliva sample and a blood sample.

If drivers are deemed legally drunk or impaired by drugs, they will be offered a hotel room, a taxi ride home or a tow truck. If they refuse, a law officer will be brought in to make the same offer. In more than 30,000 interviews since 1996, the survey company said there hasn’t been a single DUI arrest.

Those who complete the full survey are given $60.

The data-gathering locations will be moved throughout the weekend, as will the times. Electronic road signs, sandwich boards and banners will draw attention to the surveys and underscore that the process is voluntary.

Impaired driving deaths have dropped in Washington over the years. Even so, it remains the state’s top cause of fatal collisions with 1,160 deaths between 2008 and 2012.

“This roadway survey will give vital information about our impaired driving problem,” said Darrin Grondel, director of the state traffic safety commission.

Washington voters approved Initiative 502 in 2012. The new law regulates the sale of small amounts of marijuana to people 21 and older.

Eric Stevick: 425-339-3446, stevick@heraldnet.com