There are several types of roadblocks and they're all quite different from one another:
Also known as DUI Checkpoints or sometimes Driver's License Checkpoints, these are the most common roadblocks you might encounter. They function as a general purpose investigatory tactic in which police get a good hard look at passing motorists by detaining them briefly. A roadblock stop is quick, but it gives police a chance to check tags and licenses, while also giving officers a quick whiff of the driver's breath and a chance to peer into the vehicle for a moment.
Remember that your Constitutional rights still apply in a roadblock situation. Though police are permitted to stop you briefly, they may not search you or your car unless they have evidence against you or you agree to the search. Bear in mind, however, that if you're driving under the influence, your Constitutional rights provide very little protection in this situation.
Since the Supreme Court's ruling in Illinois v. Caballes police also have more leeway to use drug-sniffing dogs in roadblock situations. Unfortunately, the Constitution provides very little protection against this. There's no need to waive your rights simply because dogs are present, but be advised that your legal options are limited if you're arrested as a result of a dog sniff during a roadblock. Keep this in mind when deciding who or what to bring with you in the car.
Also keep in mind that police closely monitor cars approaching the roadblock. You're not likely to have any success evading an upcoming roadblock.
Sobriety Checkpoints are generally permitted by the courts, but only if conducted properly. If you're arrested at a police roadblock always consult an attorney before confessing or agreeing to a plea bargain. There might be some legal options that your lawyer can pursue.
This Utah motorist did an excellent job handling a DUI checkpoint in his home state:
Sometimes police will set up temporary roadblocks after a serious crime occurs. The purpose of emergency checkpoints is to capture suspects or to identify possible witnesses. In this situation, police will often allow you to pass through once they confirm that you're not the person they're looking for. Of course, police are free to arrest you for minor crimes even if they're investigating something more serious.
If a serious crime occurs in your area, keep in mind that more police will be on the streets. Officers are often required to work longer hours during emergency periods, which can make them tense and irritable. Use caution in such situations even if you haven't done anything wrong, and remember that dealing with emergencies is something we want our police officers to do.
Checkpoints Near the Border
Police sometimes set up checkpoints near national borders. These are similar to other checkpoints in that officers may ask questions and check your documents. Police may try to intimidate you in consenting to a search, but remember that being near a border is not the same as crossing it. You have a right to refuse searches at these checkpoints just like the others.
Drug checkpoints are a trap! The Supreme Court has ruled that random checkpoints for the purpose of finding illegal drugs are unconstitutional. However, police sometimes put up signs warning drivers of up-coming drug checkpoints and instead pull over people who make illegal u-turns or discard contraband out the window. If you see a sign saying "Drug Checkpoint Ahead", just keep driving and don't panic. If there's a rest area following the sign, DO NOT pull into it. If you do, you'll find yourself surrounded by drug-sniffing dogs.
Police Departments, especially in the Mid-West, have been pushing their luck with this tactic, so if you encounter anything resembling an actual drug checkpoint, please contact that state's ACLU Chapter. Similarly, if you're arrested as a result of a real or fake "drug checkpoint", you must contact an attorney to explore your legal options.