They can test if a person has THC in their system but if they have it doesn't they are stoned
However it would be pretty obvious if some one was stoned enough that it effected there driving, in the USA they do
Field Sobriety Tests to preliminary determine if a person is under the influence of drugs and alcohol.
You would be surprised at how many people from all walks of life (seriously all walks of life) use cannabis, just like alcohol most people use it responsibly and don't drive or use machinery when under the influence. That some people abuse drugs and alcohol is not a reason to deny them to those that are responsible.
Did you read the article Raider posted? here it is
https://www.theverge.com/2019/2/5/1...-fatality-deaths-transportation-public-health
