[cletus]

I believe it serves several purposes:

1. It hops to discourage people from drinking and driving in the first place by planting the idea that there will be checkpoints. The best time to avoid DUIs is before the driver starts drinking. Ideally, they'll make alternative arrangements to get home from wherever it is they're going rather than taking the car and figuring it out after;

2. To enable those capable of driving to avoid the checkpoints. The premise for this is that those who are drinking and driving either are capable of making this kind of rational forethought or they're not and the police are largely interested in catches those that aren't; and

3. Possibly to divert those that are borderline cases from driving through areas where they might cause the most damage if something does go wrong.

I know people like to see speed traps and DUI checkpoints as cynical revenue-raising initiatives but you'd be surprised to learn that some people just don't want others to act irresponsibly by driving several tons of metal at high speed while impaired, possibly harming or killing themselves or others.