Studies show that illicit drugs impair the skills needed to operate a motor vehicle safely because they affect coordination, reaction time, muscles, vision and the ability to judge distance and speed. In the case of marijuana, thinking and reflexes are slowed, making it hard for a driver to respond to sudden, unexpected events. The lack of ability to brake quickly, stay in a lane, maintain correct speed and proper distance between cars, greatly increases the risk of a serious accident.
Methamphetamines can cause lapses of attention, disorientation, a lack of coordination and overconfidence in driving skills. The latter leading to increased risk taking and aggressive driving, which is extremely dangerous. As the effects wear off, drowsiness and fatigue, difficulty concentrating, irritability and depression may continue to impair driving ability.