The following description was taken from Cannabis & Driving by ICADTS :

The aim of this fact sheet series is to review the scientific evidence on cannabis and driving to provide objective information and inform the development of legislation to manage cannabis-impaired driving. The International Council on Alcohol, Drugs & Traffic Safety (ICADTS) aims to address road safety, with a focus on preventing and reducing traffic crashes caused by driving under the influence of cannabis. This approach demands a different approach from drug control strategies designed to reduce illicit cannabis use in the general population.
In jurisdictions where cannabis is entirely illicit, drug control might be prioritized over road safety. That is, a positive result on a biological test may result in a penalty for driving under the influence, regardless of the degree of impairment. However, as an increasing number of jurisdictions no longer prohibit cannabis use, we need to develop effective road safety policies that distinguish between cannabis-impaired driving and the prior use of cannabis only. This is especially pertinent with the increasing use of cannabinoid products as prescription medicines (i.e., medical cannabis).