Roadside drug testing.

This week the New South Wales government announced that it had achieved it's first court conviction for driving while affected by a drug - detected by it's new roadside testing regime.
This system is new - and very mysterious. Little has been divulged except that the state has a single mobile testing unit housed in a bus - and a small squad of officers trained in detection techniques. It was stated that the squad would not be conducting normal roadside testing - similar to the alcohol testing units - because each test took about twenty minutes and was thus unsuitable for mass screenings.
The government intended to target special events - such as rave party exits and lay-overs for heavy vehicles whose driver's were notorious drug users.
What concerns many is the regimen involved for testing. The most common drug used by many is Marijuana and it is a fact that residue from it's use lingers in the body for weeks - and in some cases months. As the drug is illegal, could it be that a conviction would be sought for it's mere presence - which would indicate that the person tested had conducted an illegal act to have that trace in his or her body ?
The purpose of detection was supposed to be to remove drivers who were behind the wheel of a car with a level of drug intoxication that made them unfit to drive - similar to those over .05 for alcohol. So far, no level of safety has been mentioned where the presence of Marijuana is detected.
Obviously there is a desperate need to remove drivers who get behind the wheel with a dangerous level of alcohol or drugs in their body - but we need to know what testing is involved for drugs - how levels are measured - and what levels constitute illegality leading to arrest and a court appearance.
So far that has not been forthcoming. Now that the first conviction has occurred a frank appraisal of what system we have in place is essential !