The blood alcohol level 0.02 was mentioned several times in the article. Why? Is that the lowest accurately measurable level? Is it the legal limit in many places?

Here, the limit is 0.08, just less than two drinks, making true the old adage: "One's enough (OK to bike home), two's too many (get a ride or walk), three's not nearly enough (pass out on the floor and bike home the next day).