Whereas alcohol intoxication leads drivers to underestimate their impairment (resulting in speeding and other forms of increased risk taking), marijuana generally leads drivers to overestimate their impairment (resulting in slower driving speeds despite explicit instructions to maintain a particular speed). However, the combined effects on driving ability of marijuana and alcohol do not nullify each other. Evidence suggests that impairment as a result of both substances is greater than either alone (64) and may be more than additive (71).
