I agree wholeheartedly with your assertion that chemical tests are worse than useless. There have been various efforts to instate policy of placing people who test positive for marijuana under DUI arrest.

Everyone cheers that it's a good thing, but it's completely unfair to the responsible marijuana user. The active ingredient is detectable in the body for several weeks after the period of last ingestion. This doesn't imply that the user is impaired or even high in the slightest, yet it would be enough to warrant a DUI under such conditions. Such a policy is simply discrimination and does nothing to make the roads safer.