In fact, I believe random breath testing is a way to overcome the very problem you talked about. To set the scene, prior to the introduction of RBT in Australia, the police would breath test people using their discretion. This tended to mean that they used to surveil or hang around drinking venues and look for telltale signs of drivers being impaired. Also—and, having studied the direction of random breath testing, I've spoken to police officers who confirm this—there was a tendency to keep an eye out particularly for older vehicles, vehicles driven by younger people, and in many cases vehicles that weren't considered to be driven by more affluent people.
What happened under random breath testing was that the guideline for the police was that they were to pull over all drivers randomly. What that means, particularly in the booze bus operations, is that, as long as capacity can take it, the police actually pull over everyone in the traffic stream, so they aren't showing any discretion at all. They are testing all drivers.
The practice varies a little from state to state. In the state of Victoria, for example, they've been known to shut down all freeways and breath test everyone. In cases where there is very heavy traffic, there will be a selection process occurring, but what happens is that they will select a cohort of vehicles or a group of vehicles coming along, and it's very rare for them to wave a particular vehicle on.
Overall, having observed RBT operations and spoken to police, I think the key essence of it is to breath test people on a random basis without showing any discretion.