Marijuana impairs skills, performance and perceptual processes, including vision, attention and tracking behaviour-all-important components of driving performance.

This statement is true.

Explanation:

Marijuana is a drug which tends to "calm down" an individual and relax him/her. It is often related to a sensation of calmness and tranquility, diminishing an individual's motivation. It also affects the individual's perception.

Some secondary effects include:

Less coordination, thus less capacity to carry out activities which require it. Vision impairment. Learning impairment: the individual will be less concentrated, less attentive and less vigilant.