The title seems misleading. People with marijuana in their bloodstream != stoned drivers. Marijuana use is detectable for a long time past the point of impairment and the data only reflects that they HAVE used marijuana vs being stoned at the point of driving.

Maybe instead of breathalysers and drug detection kits we should have standardised attention/reaction tests to determine if someone is safe to drive (you could probably even run them on a tablet). This would also weed out (pun intended) tired drivers.