Yeah, based on the link in your first comment the answer is that there is basically no way to test impairment without just giving people impairment tests. So even though we may have moved to this because saliva tests are a bit dodgy, it's probably not much better.

Honestly, I think we should just throw loads more money into getting self-driving cars sorted. Well, I guess there is already loads of money being thrown at it so we should wait it out. Then we can just ban driving and solve the whole problem.