If, as is being reported it is true that the Government plans to introduce roadside drug testing with saliva and the police will merely have to prove that you have taken drugs at some point to fine you then it is wholly, and completely the wrong approach to dealing with potheads and pillheads that stupidly get in their cars.
The reason is because if someone were to have a joint tonight then the saliva test would remain postive in days to come up to almost a week and potentially more. Clearly though they are not under the influence of the drug anymore. As such there has to be additional tests that can be done, just like with booze.
Currently, for booze, it makes sense to have a hangover time for being under its influence. If I get steaming drunk tonight and then get up at 6am tomorrow morning I may feel fine but the effects will still be there in my system impaairing me. However, if I got stoned and then didn't drive again until next Wednesday they would not be, that is a fact.
Illegal substances mainatin a presence in the body for a long period of time but do not influence the user for the time they mainatin their presence. That is a simple fact. Tetra Hydra Cannabinol can remain present in the system for up to a month after it's taken, but the idea that person is stoned under its influence for that time is nuts.
The obvious solution I would say is to have the ability for someone failing a saliva test to have a blood test and urine test as well that can determine the level of a given drugf and make an assessment according. Simply having the black and white of "is present/is not present" at the roadside will end up punishing people when they have not been dangerously driving.
Please note before anyone says that people should be breaking the law and taking illegal drugs anyway. Taking controlled substance is not illegal, it is the possession of them outside the person (in a pocket etc) that is. The presence of drugs in someones system does not mean that they must have been guilty of possession as well (they could have been spiked).