It's easy to target people who are engaging in unnecessary actions at the expense of driving capability. Drinking, using drugs, and texting fall under this category. People are much less accepting of punishments for things seemingly outside of personal control. Imagine the embarrassment for law makers when some mothers' advocacy group attacks them for banning all new mothers from driving (except those rare exceptions who function well on <4 hours of sleep.) Or the backlash for hardworking people getting arrested regularly because their multiple jobs keep them from getting an amount of sleep that makes driving permissible.
I agree that the "why" isn't that important (or at all), but what is the alternative?
It could hypothetically be as simple as a video game driving test. It could be some other abstract measurement of reaction time.
I don't understand the question. A driving skills / reaction time test, is an alternative.
The evidence of the stuff stays in your system much longer than the impairment. There is also the issue of "tolerance" and or how the amount of substance present in urine or blood relates to the supposed impairment.
>So now you effectively have no way to prove whether or not someone was breaking that law.
But you can still perform an objective test to determine if someone is currently impaired. They need to design and validate a motor skills and reaction time test. I'm not sure why anyone cares more about why someone is impaired than they do about whether someone is impaired.