Mother Jones surveys the growing problem of marijuana-impaired drivers - a problem exacerbated by the lack of tests for determining how badly impaired these drivers are.
In all 50 states, if you're pulled over on suspicion of driving under the influence, you must submit to a Breathalyzer test or face arrest and possibly a blood test. Yet cops lack anything like a Breathalyzer for THC, and studies have shown that the field sobriety test widely used by police departments correctly fingers stoned drivers only about 30 to 50 percent of the time; drunks are detected 80 percent of the time.
It's a serious gap, because even relatively small amounts of marijuana severely compromise driving ability.
Road tests and driving simulator studies have found that the more weed drivers inhale, the worse they do at essentials such as staying in their lanes, responding to sudden hazards (like a dog running into the street), and multitasking—for example, reading street signs on a twisty road while avoiding oncoming traffic. On average, drivers with blood THC levels equal to or in excess of 5 ng/ml cause crashes at 2.7 to 6.6 times the rate of sober drivers, and getting into the driver's seat less than an hour after smoking a joint nearly doubles your risk of getting into a crash.