What if you're tired? If you've taken no drugs, but worked twenty hours in a row? You WILL fail an impairment test. But you've done nothing wrong. Maybe you shouldn't be driving. But then, the state SHOULD be providing affordable and comprehensive public transit. Infrastructure is the government's #1 job, not that they seem to take it very seriously.
If you're in Europe, you can tune this out. Even in most American cities public transit is abysmal. In Boston for example, the subways shut down at midnight. For reference, the bars close at 1AM. That's in a major city. Most Americans don't live in the cities. And most suburban and rural areas don't even have taxi services available. In most of the US, rhetoric aside, driving isn't a privilege: Driving is a necessity.
Oddly, American towns and cities did had good public transit a hundred years ago. Back then, even most small towns had trolley services. Then a conspiracy destroyed public transit. It's OK though; after making millions destroying public infrastructure and lobbying against light rail, GM had to pay a $5000 fine. For more details, watch "Who Framed Roger Rabbit?" It covers the basic story pretty well. The real life bad guys were a shade more cartoonish than Christopher Lloyd though.
And then there is the question of enforcement. Would the testing be done in the same, "at the whim of the guy with the badge" approach we have now? We don't inspect or enforce pretty much any public health issues in the US. Food safety, Drug safety, traffic safety, etc.: All public safety in the US is set up to assign blame after stuff hits the fan. It didn't used to be like this either.
How do you objectively test for "situational awareness"? I don't know of ANY way to do it, that isn't completely at the whim of the person conducting the test. Most cops should not be taken at their word.
Right now, there are huge scandals in Massachusetts over these exact two things. One over a police lab making up evidence, falsifying data and generally doing bad things; for more than a decade. And second a failure to oversee a compounding center, and just generally assigning blame later after many people contracted fungal meningitis from contaminated shots. And the only reason it was exposed was that one doctor took on themselves to track down the cause of an odd outbreak.
The test should be for reactions and situational awareness. If you fail for any reason, then you should be prevented from driving. If you fail and also have been taking drugs that are known to cause this kind of impairment, then you might get some extra penalty.
It's a common brush-off answer. But it ignores reality completely. If you can't balance on one leg and touch your nose, while reciting the alphabet backwards... you fail. If you refuse to walk a straight line backwards in icy, sub-zero conditions... you fail. It doesn't matter that you have artificial hips, knees, or limbs.