How does marijuana smoking affect driving?

Marijuana affects many skills required for safe driving: alertness, concentration, coordination, and reaction time. These effects can last up to 24 hours after smoking marijuana. Marijuana use can make it difficult to judge distances and react to signals and sounds on the road.

There are data showing that marijuana can play a role in crashes. Studies show that approximately 6-11 percent of fatal accident victims tested positive for THC. In many of these cases, alcohol was detected as well. When users combine marijuana with alcohol, as they often do, the hazards of driving can be more severe that with either drug alone. In a study conducted by the National Highway Traffic Safety Administration, a moderate dose of marijuana alone was shown to impair driving performance; however, the effects of even a low dose of marijuana combined with alcohol were markedly greater than for either drug alone.

In one study conducted in Memphis, TN, researchers found that, of 150 reckless drivers who were tested for drugs at the arrest scene, 33 percent tested positive for marijuana, and 12 percent tested positive for both marijuana and cocaine. Data also show that while smoking marijuana, people show the same lack of coordination on standard "drunk driver" tests as do people who have had too much to drink.