Over at The Huffington Post, Russ Belville notes that the Obama administration's new drug control strategy urges states to adopt "per se" laws for driving under the influence of drugs (DUID):

Fifteen states have passed laws clarifying that the presence of any illegal drug in a driver's body is per se evidence of impaired driving. ONDCP [the Office of National Drug Control Policy] will work to expand the use of this standard to other states and explore other ways to increase the enforcement of existing DUID laws.

Under these laws, a driver who has marijuana metabolites in his urine is automatically considered impaired, even though he probably isn't. Because marijuana metabolites can be detected in urine long after the drug's effects have worn off (up to two weeks for occasional users, months for frequent users), a "zero tolerance" DUID standard punishes pot smokers who have not harmed or endangered anyone. It is a way of imposing an extra punishment for marijuana use under the guise of traffic safety, sending unimpaired drivers to jail as public menaces because they smoked pot days or weeks before getting behind the wheel. It's like arresting someone for drunk driving on a Wednesday because he drank a few beers the previous Saturday. And it is completely unnecessary, since a standard based on THC in the blood (analogous to the standard for DUI) would be a much better indicator of impairment. In fact, since "the presence of metabolites for THC tells you the body has already broken down the THC," Belville says, "you could actually call a urine screening for metabolites a non-impairment test!" This is exactly the sort of mindlessly draconian policy that a president with any interest in dialing back the war in drugs would avoid.

I explained the problems with "per se" DUID laws in a 2006 column.