The effect of prior announcement of speed or alcohol checks via radio or other media has not been studied. Such prior announcements enable people to avoid fines by adapting their behaviour on the relevant road section or to choose an alternative route to avoid the police check. The latter seems to happen, for instance, when social media are used to warn about ‘alcohol traps’. Such large-scale, static alcohol checks may therefore have become less effective. That is why the police focus on individual checks.
Prior police check announcement may, however, also have a positive effect. If people often hear such announcements, they get the impression that police are often active, and at different road sections. This will lead to a higher subjective probability of detection, which is one of the crucial elements of an effective enforcement policy (also see the question What does traffic enforcement effectiveness depend on?).
