CHARLESTON, S.C. (WCBD) – Law enforcement agencies across the country routinely conduct DUI checkpoints to discourage impaired driving, especially during major holidays. These checkpoints always come with prior warning, leaving some to question whether they are actually effective at catching offenders.    The answer to why local agencies publicize traffic checkpoints in advance is straightforward: It’s required by law. The law stems from a 1990 Supreme Court case brought by a Michigan driver in which he argued the Michigan State Police Department’s sobriety checkpoint program violated his Fourth Amendment protection against unreasonable search and seizure. In a 6-3 decision, the Court held that checkpoints are constitutional because the state’s interest in public safety outweighs the minor intrusion on the stopped driver's privacy. The Court also found, however, that checkpoints must follow certain guidelines to be constitutionally valid. South Carolina is one of 38 states and the District of Columbia that allows DUI checkpoints. 12 states do not. Charleston Police adds extra enforcement on New Year’s Eve No state statute directly authorizes or prohibits DUI checkpoints in South Carolina. Instead, the state relies primarily on case law to regulate the practice. To be considered legal, the date and location of a checkpoint must be publicized in advance and set up in a way that minimizes inconvenience to drivers. Officers must also clearly identify themselves and follow a uniform procedure for screening drivers. But, do checkpoints actually work? Research shows there has been a greater reduction in incidents related to drinking and driving after announcing traffic checkpoints. A 2014 review of studies found that checkpoints reduce alcohol-related fatal crashes by 9%, according to the National Highway Traffic Safety Administration. What is the penalty for drunk driving in South Carolina? South Carolina law states a person cannot drive if their faculties are “materially or appreciably impaired” by alcohol. The legal BAC limit is 0.08% for drivers 21 and older. Penalties for a DUI conviction can depend on several factors, including BAC level, previous offenses, and whether any damage, injuries, or death occurred. Violators could face fines, imprisonment, and/or loss of driving privileges. Any person convicted of drunk driving in South Carolina will have a temporary breathalyzer installed in their vehicle under a new law that went into effect in May.