Cannabis

I've used marijuana for over ten years. Should it be legal? Yes.
However, I strongly disagree that cannabis is harmless for every person and under any circumstance.
There are strong arguments for the legalization of marijuana, that it 'makes driving safer' is not one of them.


Some studies that contradict the study you have posted:

1. Cannabis Effects on Driving Skills
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3836260/
Research studies have shown negative effects of marijuana on drivers, including an increase in lane weaving and poor reaction time and attention to the road. Use of alcohol with marijuana made drivers more impaired, causing even more lane weaving (Hartman, 2013).


2. Dose related risk of motor vehicle crashes after cannabis use
http://www.drugandalcoholdependence.com/article/S0376-8716(03)00284-9/abstract?cc=y=

The role of Δ9-tetrahydrocannabinol (THC) in driver impairment and motor vehicle crashes has traditionally been established in experimental and epidemiological studies. Experimental studies have repeatedly shown that THC impairs cognition, psychomotor function and actual driving performance in a dose related manner. The degree of performance impairment observed in experimental studies after doses up to 300 μg/kg THC were equivalent to the impairing effect of an alcohol dose producing a blood alcohol concentration (BAC) ≥0.05 g/dl, the legal limit for driving under the influence in most European countries.


3. Psychoactive substance use and the risk of motor vehicle accidents
http://www.sciencedirect.com/science/article/pii/S0001457503000848

The driving performance is easily impaired as a consequence of the use of alcohol and/or licit and illicit drugs. However, the role of drugs other than alcohol in motor vehicle accidents has not been well established. The objective of this study was to estimate the association between psychoactive drug use and motor vehicle accidents requiring hospitalisation.