&gt;The stuff stays in your system too long<p>The <i>evidence</i> of the stuff stays in your system much longer than the impairment. There is also the issue of &quot;tolerance&quot; and or how the amount of substance present in urine or blood relates to the supposed impairment.<p>&gt;So now you effectively have no way to prove whether or not someone was breaking that law.<p>But you can still perform an objective test to determine if someone is currently impaired. They need to design and validate a motor skills and reaction time test. I&#x27;m not sure why anyone cares more about why someone is impaired than they do about whether someone is impaired.