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Alcohol and Impaired Driving

What does blood alcohol concentration measure? 
Blood alcohol concentration (BAC) describes the amount of alcohol in a person’s blood expressed as weight of alcohol per unit of volume of blood. For example, at 0.10 percent BAC, there is 100 mg of alcohol per 100 ml of blood. For most legal purposes, however, a blood sample is not necessary to determine a person’s BAC. It can be measured much more simply by analyzing exhaled breath.

What is the effect of alcohol on driving skills and crash risk? 
Alcohol causes poor judgment. It slows your reaction time and your ability to concentrate and make decisions. It impairs your eyesight. If your BAC reaches 0.08, you are three times more likely to have a traffic collision than if your BAC were zero. If your BAC reaches 0.25 percent, your chances of having a traffic collision are 25 times greater.

How many drinks does it take to reach significantly impaired BACs? 
The effects of alcoholic drinks vary greatly because of the rate of absorption and BACs attained vary from person to person due to such factors as weight, amount of fat tissue, and stomach contents. Alcohol affects small or thin people sooner and stays in their systems longer than it does with heavier or larger people because the alcohol is more concentrated in the smaller person’s bloodstream. 

Are beer and wine less impairing than hard liquor? 
Impairment is not determined by the type of drink but rather by the amount of alcohol ingested over a period of time. There is a similar amount of alcohol in such standard drinks as a 12-ounce serving of beer, a 4-ounce glass of wine, and 1.25 ounces of 80 proof liquor. Beer is the most common drink consumed by people stopped for alcohol-impaired driving or involved in alcohol-related crashes.

Do alcohol related crashes differ by gender?
Crashes involving men are much more likely to be alcohol related than those involving women. Among fatally injured male drivers of passenger vehicles in 2000, 34 percent had BACs of 0.10 or more. The corresponding proportion among women was 18 percent. Alcohol involvement in fatal crashes is highest for men ages 21-40.

When do alcohol-related crashes occur? 
They happen at all hours, but alcohol involvement in crashes peaks at night and is higher on weekends than on weekdays.

What’s the goal of alcohol-impaired driving laws? 
Many people think the principal goal of such laws is to arrest and punish the drivers who put everyone else at risk. But arrest and punishment of offenders is a secondary objective. The most important objective is for the law to be a deterrent so that police find no alcohol-impaired drivers to arrest.

Why is deterrence so important and how can it be achieved? 
Most impaired drivers are never stopped. Others are stopped, but police miss signs of impairment. It has been estimated that as many as 2,000 alcohol-impaired driving trips occur for every arrest and that, even when special impaired driving enforcement patrols are conducted, as many as 300 trips occur for each arrest. Because the police cannot catch all offenders, the success of alcohol-impaired driving laws depends on deterring potential offenders by creating the public perception that apprehension and punishment of offenders is likely. The key to creating this perception is enforcement. Enforcement efforts must be well publicized and create a realistic threat of apprehension.

Who can be stopped for impaired driving?
Although police cannot stop and test individual drivers without cause, they can investigate any driver who, based on established criteria, appears to have been driving while impaired by alcohol. Most alcohol-impaired driving arrests are made by officers on routine patrol who discern signs of impairment after stopping a driver for an ordinary traffic violation.

How do checkpoints work? 
Police can use checkpoints to stop drivers at specified locations to identify impaired drivers. All drivers, or a predetermined proportion of them, are stopped based on rules that prevent police from arbitrarily selecting drivers to stop. In South Carolina, law enforcement officers conduct public safety checkpoints, checking for such things as driver’s license, vehicle registration, proof of liability insurance, and obvious equipment violations in addition to looking for impaired drivers.

Are checkpoints constitutional? 
The U.S. Supreme Court has held that properly conducted checkpoints are legal under the Constitution.

What is implied consent? 
This refers to statutes in which consent for a driver to submit to BAC testing is provided for by statute, not expressly by the driver. Any person operating a vehicle in South Carolina is considered to have given consent for testing of breath, blood or urine for purposes of determining the presence of alcohol or drugs or both in the person’s system, if alleged to have committed a violation.

What is administrative license revocation (ALR)? 
This refers to statutes in which a person’s driver’s license may be automatically suspended for certain violations of law. In South Carolina, any person who refuses to consent to BAC testing or who has a BAC reading of 0.15 percent or higher (0.02 percent for persons under 21) will have his/her driver’s license automatically suspended.

What is illegal per se? 
This refers to statutes which make it illegal to operate a motor vehicle with a certain BAC level. In South Carolina it is illegal to operate a motor vehicle with a BAC of 0.08 percent or higher.

What is zero tolerance? 
Persons under the age of 21 may not legally purchase, possess or consume alcoholic beverages. Zero tolerance refers to statutes which make it illegal for persons under the age of 21 to operate a motor vehicle with any measurable amount of alcohol in their system. Under South Carolina’s zero tolerance law, it is unlawful for any person under the age of 21 to operate a motor vehicle with a BAC of 0.02 percent or higher.