Look at this study:
http://pages.uoregon.edu/bchansen/MML_Alcohol_Consumption.pdf
It looks like VERY responsible science and statistics. It explains:
“[unlike alcohol] neither simulator nor driving-course studies provide consistent evidence that these [marijuana] impairments to driving-related functions lead to an increased risk of collision (Kelly et al. 2004; Sewell et al. 2009)”
And Figure 2 on page 55 tells the whole story.
Maybe they so blissed out they never leave their driveway, man.