Due to the limits of drug detection technologies and the lack of a consensus threshold to define impairment, doing tests for drug impairment is challenging. .08 g/mL blood alcohol concentration is the threshold of impairment for drunk driving in the United States. But for driving while under the influence of drugs, there is no comparable national standard. People respond to drugs differently each time. Additionally, marijuana and other drugs can linger in the bloodstream for weeks, making them visible in roadside tests even if they no longer cause impairment.