@ @Bastards et al

No field drug test is necessary to detain or prevent impaired drivers from being "allowed off scot free". An officer does not have the burden of identifying with scientific precision what substances you are under the influence of, only whether your mental state meets field test standards deemed acceptable for operation of a motor vehicle, a discretionary decision whether you are acting recklessly. Positive identification of drug use is only required for the specific, secondary charge if that second activity is also illegal.

They don't usually just test "for alcohol", they test for a specific BAC % which is defined by law as a limit considered an excessive impairment. That constitutes a violation of law in addition to possible reckless or other motor vehicle operation, violations.

Certainly it is no less fair or right to hold users of other (both illegal and legal) drugs to the same standard, that they not be excessively impaired while operating a motor vehicle. The problem is that there is no definition in law as to a quantity (per blood or body mass?) considered to cause excessive impairment, that in most 1st world countries evidence that you have at some point in the past consumed drugs is not an offense that is prosecuted unless you are impaired at the time which caused the suspicion and subsequent testing.

Having metabolites of drugs in your sweat cannot determine like a breathalyser or blood test does, whether you are currently impaired by the drug to any particular extent if at all at that point in time.

The alcohol tests are testing for presence of alcohol, not the metabolites from it. A similar test for alcohol might be one where your blood test indicates you drank a few beers at some point between the administration of the test and a few weeks ago. Would you like to be cited for being drunk a couple days prior to being stopped and tested regardless of whether you were even driving a couple days ago?

in the UK they tend not to get off scot free for drug driving it's pretty easy for the police to tell a drugged up person if they've stopped them.

Sadly recreational drug use is illegal (illogical in view of many rec drugs if alcohol and tobacco use is legal) I tend not to smoke draw any more, only at the odd party when somebody else has some and is offering it around (which is about 5 times in the past 2 years) but when I was a yuuf me and my lads smoked like chimneys, you only had a few choices, alcoholic, coke head, pot head, or woman beater (often reserved for alcoholics.) And having been brought up in an alcoholic house hold where my old man beat my mother and my mother could on occasion be found past out of the kitchen floor, I'm a big believer in the pros of E's and cannabis over Alcohol. Though alcohol is the only legal inhibitor I can get, so alcohol it is.