Those roadside drug tests are a joke. I know from personal experience.
I won't lie, I've smoked green stuff. In fact, one time about 3 years ago I had some when the other half comes in and wants me to go to the chemist for a script she needed.
On the way I got pulled into a drug and alcohol testing station. Blew negative for alcohol (rarely drink) and got tested for drugs.
I passed despite having had a cone less than 30 minutes earlier