Just read this article (https://www.sciencedaily.com/releases/2019/08/190827123239.htm) about a breath-test device being trialed in the US and Canada that can detect THC molecules in a sample . How it might be deployed is too early to say, but its worth remembering that their LE don't conduct mass RBT's like here in Oz. As with other detection systems in use, there is no 'sobriety scale' attached - rather an indication of how soon before testing that cannabis has been smoked. Essentially, its a scaled-up detector that still does nothing more than prove you've broken the law by having THC in your system if you get pulled over whilst driving. It doesn't pretend to do anything else, so even in more liberated parts where cannabis use is legal or decriminalised, there is still a price to pay if you like a choof and have a driver's licence.