Opponents of Proposition 19, California's marijuana legalization initiative, are falsely claiming it would allow people to drive while stoned. In a June Sacramento Bee op-ed piece, Bishop Ron Allen of the Greater Solomon Temple Community Church warned:

If this proposed initiative passes, California drivers will be able to operate a car while under the influence of marijuana. The initiative states smoking marijuana while driving is impermissible, but it would be perfectly legal to smoke or ingest marijuana immediately prior to driving. And because marijuana stays in the body so long, police officers will have virtually no way to prove if someone just ingested marijuana 10 minutes ago or 10 hours ago. Unlike with alcohol, there is no current test to show the level of marijuana intoxication. All authorities can currently do is test for the presence of marijuana. If this initiative passes, it is perfectly fine to have marijuana in your system at any time—even while driving a school bus, taxi or light-rail train.

This week a Pasadena Star-News editorial picked up the same theme:

The ballot sponsors "forgot" to prescribe an action level for driving under the influence. This poorly written law would release chaos on the CHP and other law enforcement agencies. How can they test a driver when there's no standard?

In fact, Prop. 19 does nothing to change California's law against driving under the influence of drugs (DUID). Like most states, California defines DUID as operating a motor vehicle while impaired by a drug other than alcohol. You are deemed to be DUID if "your physical or mental abilities are impaired to such a degree that you no longer have the ability to drive with the caution characteristic of a sober person of ordinary prudence under the same or similar circumstances." Unlike with alcohol, there is no "per se" rule that says you are presumed to be impaired if a test shows that the concentration of the drug in your blood exceeds a predetermined threshold. To make its case, the prosecution has to present evidence of impairment along with evidence of drug consumption. We can argue about whether this is a good standard; it seems pretty reasonable to me, although it might make sense to set drug-specific blood thresholds based on evidence tying them to impaired driving ability. Either way, this is the standard that the California legislature picked for all drugs other than alcohol, whether legal or illegal. The same complaints that Bishop Allen and the Pasadena Star-News (along with other newspapers) raise in opposition to legalizing marijuana suggest that all potentially driver-impairing drugs, ranging from over-the-counter antihistamines to oxycodone, should be banned to prevent traffic accidents.

Fifteen states have adopted a "zero tolerance" per se standard for marijuana and other illegal drugs. Under that standard, which the Obama administration wants other states to copy, a driver is presumed to be impaired if any amount of a prohibited substance can be found in his body. Allen inadvertently makes the case against this approach when he notes that traces of marijuana can be found in a pot smoker's body long after the drug's effects have worn off. This fact, combined with the scientifically unjustified distinction between legal and illegal substances, shows that "zero tolerance" DUID laws are a way of punishing drug users (through license suspension, fines, and jail time) in the guise of public safety.

Similar issues are raised by workplace drug testing. Here are the relevant provisions of Prop. 19:

This Act is not intended to affect the application or enforcement of…any law prohibiting use of controlled substances in the workplace or by specific persons whose jobs involve public safety…. No person shall be punished, fined, discriminated against, or be denied any right or privilege for lawfully engaging in any conduct permitted by this Act…Provided however, that the existing right of an employer to address consumption that actually impairs job performance by an employee shall not be affected.

The Los Angeles Daily News complains that Prop. 19 "precludes workplace drug testing by saying that employers can address only 'consumption that actually impairs job performance by an employee.' And what does impairment really mean, anyhow?" While businesses should be free to adopt whatever drug policies they deem appropriate, any sensible, cost-conscious employer, unconstrained by the government's arbitrary pharmacological distinctions, surely would ask what impairment means and what drug testing accomplishes. If, as in the case of marijuana, a positive urinalysis result is not a reliable indicator of on-the-job use or impairment, what exactly is the point, other than penalizing pot smokers qua pot smokers? Assuming that Prop. 19 imposes a requirement that drug testing have some relationship to safety or job performance (and I'm not sure it does), it would be objectionable on libertarian grounds, but it would hardly translate into "a clear right to smoke on the job," as the Daily News claims. A rule against smoking pot in the workplace, like a rule against drinking in the workplace, can easily be justified by reasonable job-related concerns.