Australia uses saliva tests to perform roadside THC, Methylamphetamine and MDMA tests. My understanding is that the Netherlands also performs similar roadside tests, and Canada is piloting doing so.
You're forgetting it's also what most people are on, which is the reason why it's the most tested for. 3 million deaths caused by alcohol every single year according to the WHO! It's almost like some sort of pandemic, only it's been going on forever and society isn't in the slightest way bothered.
I find it so odd few see the absurdity in all these public health debates. Now we have a virus going around and people go full authoritarian, even though the majority of deaths is way above the median life expectancy in many countries. Meaning they'd not have lived much longer anyway. But drunks kill tons of random people including kids every year - no biggie.
Just to be clear I'm against prohibition, I think all drugs should be legalized. There are millions of people who have serious issues with alcoholism though, and we have to do something against that, because they cause a lot of problems, even beyond killing others. It's farcical that drugs like LSD are highly illegal, even though users aren't known to harm others, but then others like alcohol are so accepted we even arguing about allowing people to drive while impaired with them.
If there were data showing that legalizing drink driving would lead to fewer accidents I'd be absolutely willing to consider it. I've never heard of anyone seriously make such an argument though, it seems like total bunk. A professional drinker may be driving inconspicuously, until one day their slow reaction time may not be enough to not hit that person crossing the street at night.
It is incorrect, at least in the US, that "the majority of deaths is way above the median life expectancy". While people over 75 have been hit the hardest, they are less than half the deaths, and Covid has caused US life expectancy to drop by two years.
Sure, on paper.
In reality 99% of enforcement is against alcohol because that's what they have the breathalyzers, blood tests and established law to go after.
It's exceedingly rare for someone to get a DUI for anything other than booze unless they are so high they fall asleep at the wheel