(Cliff DesPeaux/Reuters)

Researchers at Dalhousie University in Halifax, Nova Scotia analyzed existing studies to tease out the relationship between marijuana use and the risk of motor vehicle collision. After winnowing a field of 2,975 studies down to just nine that met their quality and design criteria, they found that driving after recent cannabis use was associated with a nearly doubled risk of being in a major collision compared to driving unimpaired. That relationship was particularly strong in the better designed studies and those that looked at fatal accidents.

The findings comport with those of a similar study published October in the journal Epidemiological Reviews.

The findings could be used to inform campaigns against driving while under the influence of drugs, the new study notes. An accompanying editorial suggests that random roadside drug testing may not be the best approach to discouraging marijuana-impaired driving. For one thing, the editorial notes, unlike blood-alcohol content, it’s not yet clear exactly what level of cannabis-related chemicals in the blood equates to impairment. Also, the editorial argues, the public’s awareness of those roadside drug stops isn’t high enough to make the prospect of being caught a sufficient deterrent to driving under the influence of marijuana.