Sobriety checkpoints or roadblocks involve law enforcement officials stopping every vehicle (or more typically, every nth vehicle) on a public roadway and investigating the possibility that the driver might be impaired to drive. They are often set up late at night or in the very early morning hours and on weekends, at which time the proportion of impaired drivers tends to be the highest.
Upon suspicion, the stopped driver is required to exit the vehicle and take a roadside sobriety test that requires the demonstration of both mental and balance skills. If the officer determines that the test has not been passed, the driver is then required to take an alcohol breath test (often called a Breathalyzer test).
ACLU - Amicus Brief on Sobriety Checkpoints - A legal brief written by the ACLU in Indianapolis v. Edmond arguing that sobriety checkpoints and drug interdictions violation the search and seizure clause of the Constiution.