Touche. On the "no" voter thing. But like I said... feeding the sharks. I'm trying to find a middle ground for you guys but it seems like human lives come in second place to your need for total accuracy and fear of inconvenience.
It's not even close really, we aren't anywhere near being able to do that yet because we don't even know how to measure how high someone is with blood draws and the most advanced testing possible.
The problem is that you can't test for metabolites of THC because they remain in your system for weeks on end. It's because THC/metabolites are nice and soluble in lipids (fat), which means they store themselves away inside your fat reserves and don't rinse out as easy as the water-soluble alcohol. The other complicating factor is that current tests don't really tell you anything about how much of these metabolites are present in urine or saliva, they're just a strict yes/no because measuring the levels has never been necessary before (when it was still locally illegal). The only tests that can currently measure the quantities are blood draws, but then you can't go drawing blood at every traffic stop nor does the quantity of metabolites give you any real useful information. The information about quantities is useless because different people have WILDLY different perspectives when the same amount of metabolites are present, it's not like alcohol where there's a nice clean scale.
Basically you have to rely on roadside sobriety tests at the moment, because police have no other methods of detecting impairment from marijuana.
So now my car's malfunction will give police reason to pull me over, and I'd be willing to bet they'd also use it as probable cause to search your vehicle just because when you blew sober on the breathalyzer. Your car also now has a direct line to the police, meaning they have a direct line back to you. Grown adults don't need to have police constantly babysit them, and doing that won't solve any issues because it'll stretch police resources too thin to accomplish anything useful.