Yeah, but the poster above us talking about the degree to which someone is stoned, not just true/false. Relying purely on human judgement is not a good idea for something like that. Especially considering all of the abuses police commit already. If all they need to do is judge you're too stoned without any quantifying test, that's not good.
IIRC, in most jurisdictions, that's all they need to do with alcohol to arrest. Blood/breath results are additional evidence which will aid with conviction, since there is an prima facie legal limit as well as a prohibition on driving while impaired do to alcohol regardless of BAC, but most jurisdictions also prohibit driving while impaired with any substance, whether or not that substance is one for which a prima facie limit has been established.
