The deployment of the booze buses is controlled by operational police, but senior police set various targets for the amount of breath testing that should be performed and the locations where that should occur. In keeping with the principles of general deterrence, I know that the police try to achieve a broader spread of the booze bus operations across their areas of control. In fact one of the things that prevents or discourages them from parking the bus always at the one time is that drivers quickly start to learn and spread the news that the bus is parked in a particular place. One of the challenges for police is to ensure that they can keep uncertainty about where the buses are being placed. An issue there is that the larger the bus, the more constraints there are on where it can operate. There are particular occupational health and safety issues that need to be kept in mind for the police.
There are a couple of other things I should mention. Often with the big booze buses they'll use a patrol car in combination. For example, a typical style of operation is that you have the booze bus on a major road, but if there are any turnoffs as the drivers approach the bus, they'll have a patrol car parked down that street to try to create a kind of satellite effect.
Coming back to your general point, the essence is to get a widespread effect of the booze buses. This whole issue about avoiding drivers and having some predictability about where the booze buses are tends to offset the issue you've raised.