How Does Marijuana Affect Driving? Answer: Marijuana has serious harmful effects on the skills required to drive safely: alertness, the ability to concentrate, coordination, and the ability to react quickly. 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. Marijuana may play a role in car accidents. 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 have also shown 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. How Does Marijuana Affect Driving?