I'm not sure where you are going with that, but within the last couple of years here, there was at least one accident with fatalities where a drug impaired truck driver used cars in front of him to stop his truck ... with fatalities and injuries. As I recall you favor legalizing pot, but the last I knew tests will still show use weeks later, so how could you tell whether someone was impaired or used it a couple of weekends ago? Recent alcohol use and tests trend pretty well; however, it's less clear cut for drugs that result in reduced safety in the workplace.