NORML notes that a study reported in the December issue of the journal Traffic Injury Prevention found that Sweden's adoption of a "zero tolerance" law for driving under the influence of drugs was followed by a 10-fold increase in blood samples submitted for tests by police. Under a zero tolerance policy, which the Bush administration has been urging more states to adopt, it is illegal to drive with any detectable amount of a controlled substance in your blood, even if you're not impaired. Since marijuana can be detected long after its effects wear off, driver tests in the U.S. tend to catch pot smokers, whether or not they're driving while stoned. But in the Swedish study, cannabis alone was detected in just 4 percent of cases, cannabis along with other substances in another 20 percent. By contrast, 60 percent of the drivers tested positive for amphetamines. 

Which raises the question: Does the government really want to discourage drowsy drivers from taking stimulants? Since the article isn't available online, I'm not sure how police selected drivers for testing. But even if it turned that drivers involved in accidents were especially likely to test positive for amphetamines, that doesn't mean eliminating amphetamine use by drivers would, on balance, improve public safety. Since sleepy drivers are prone to accidents and people take amphetamines when they're sleepy but need to stay up, testing positive for amphetamines could be a marker for sleepiness. The important question is how many people avoid crashes by taking amphetamines that keep them awake and improve their alertness.