Police officers in a roadblock would walk up to the car and administer a roadside screening test, blown into the device. If it's truly mandatory, then they would proceed to request the subjects to stick out their tongue and draw a sample. They'd put it in, take it to the car, press the button, and wait five minutes. They'd get the results. In the case of the breath test, the results are known within seconds. It's quite simply there.
In Australia, in that situation, if they have a positive at the roadside, they then request the person to accompany them to another location. They have what they call “booze buses”. Inside the booze bus then they administer the second test, write it up, and take charge of the matter.
In Canada, we have had check stops operating across the country in different fashions. Simply, a road is blocked. It might be an access road to a highway or a major thoroughfare in downtown Calgary. I've had the fortune of being stopped in both. It's a funnel, and everyone goes through the funnel. There is no determination as to who goes one way or another. In the case of the RIDE program in Toronto, they typically ask questions of the person—for example, “Have you been drinking today?” That's to gather that reasonable suspicion. If there is no suspicion, then you go on your way.