thats bullshit, because a person can test positive for almost all illicit drugs days after they ingested them and therefore no longer under the influence. all you need is a dick-head cop whose looking to fuck wit you based on any number of 'reasons'.
Well actually in a number of US states if you have marijuana in your system and are not high you will be charged as if you were high and driving. This has been on the books for a while. And if for example you take a valium like a few days ago and then you have a car accident and they test you. Trust me. They are going to say you were high unless you have a prescription.