Oi! No getting bogged down in practicalities in this thread please!
I am under the impression that being under the influence of alcohol or drugs can actually be a mitigating circumstance for public order offences - resulting in softer sentences. Whatever though, have it whichever either way you like it has nothing to do with the legal status of the drug itself. In the case of currently illegal drugs possession is a separate charge, and it's not a crime in itself simply to be under the influence of an illegal drug in a public place, or to have taken a drug in the past.
Drunk drivers are indeed scum and this extends to everyone driving under the influence of any other drug. If the evidence suggests that (say) a 48-hour sobering-up period is required after taking X amount of drug Y before a person should drive, then legislation should of course reflect this...driving legislation. Make it illegal to take Night Nurse and drive as well if that's appropriate, as long as we have legislation that reflects the physiological effects of each substance; science- not ideology-based. Sweat swab tests, blood tests following an accident, or whatever would all continue to supplement breath tests to test for the influence of other drugs.
Alcohol is actually an example of why liberalisation is not a problem for driving legislation; it's completely legal to possess and consume as much alcohol as you like but completely illegal to drive while under the influence....in exactly the same way that drug Z could be made completely legal to possess while remaining illegal to drive under the influence of. The legal status of that drug is and would remain completely irrelevant for anything other than charges relating to its supply and possession. Drug-driving already occurs because prohibition does not work, it would not be a new result of liberalisation.