DWI Alcohol Testing
Federal law states that all motor vehicle operators with a blood alcohol concentration (BAC) of over 0.08 grams of alcohol per liter are guilty of driving while intoxicated. Motor vehicle operators may be assessed for DWIs in a variety of ways and driver's rights vary by state. All jurisdictions, however, use a combination of roadside breathalyzer testing, field sobriety tests and blood tests administered at a police station to determine whether to press charges for DWI.
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Breath Alcohol Device Testing
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Law enforcement officials use breath alcohol devices----often called breathalyzers----to estimate a motor vehicle operator's breath alcohol concentration (BrAC) in order to determine whether that operator is driving while intoxicated (DWI). The BrAC measured by the breathalyzer is then entered into a formula that determines the operator's blood alcohol concentration. Breathalyzer's do not measure blood alchohol concentration (BAC), which can only be done through a blood test. All breathalyzers used by law enforcement agencies have been approved by the United States Department of Transportation. Breathalyzer tests administered at the roadside are called preliminary alcohol screening (PAS) tests, although a breathalyzer may also be used to determine blood alcohol content at a police station. Drivers have a right to refuse PAS tests in all states other than Montana.
Breath Alcohol Testing Device Methods
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Breathalyzers are plug-in or battery operated devices with a breathing hole and electrical anode at one end. When motor vehicle operators breathe into the device, the anode converts the ethanol (molecular alcohol content) in their breath into acetic acid. The amount of electrical current produced by this reaction is then measured to determine the alcohol content in the suspect's breath.
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Field Sobriety Tests
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In addition to breath acohol device tests, law enforcement officers may ask DWI suspects to undergo a field sobriety test (FST). During a field sobriety test, suspects will be asked to perform a series of standardized tasks that are consider simple for a sober person but potentially difficult for a drunk one, including walking in a straight line and touching their hands to their nose. In all states other than Montana, drivers cannot be penalized for refusing to take a field sobriety test.
Blood Alcohol Concentration Tests
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Suspects who fail breath alcohol device tests or field sobriety tests----or who refuse these tests but who give law enforcements agents probable cause to suspect DWI----may be brought to a local police station for a blood alcohol concentration test. BAC tests are done through blood or urine samples and occasionally through a breathalyzer. Drivers with a BAC of over 0.08 grams of alcohol per liter of blood may be guilty of a DWI. Drivers may refuse BAC tests, but this will result in automatic license suspension and other penalties.
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