Drinking alcohol and driving a car do indeed not mix. Alcohol use slows reaction time and impairs judgment and coordination, the latter two of which are skills needed to safely operate an automobile. The more alcohol one consumes, the more those skills become impaired. Driving while impaired, and driving under the influence, are criminal offenses.

What is Blood/Breath Alcohol Concentration?

Blood alcohol concentration is a measurement of the amount of alcohol that is in a person’s bloodstream. Breath alcohol concentration is a measurement of the amount of alcohol in one’s breath. 

The term “blood alcohol content” is also referred to as “blood alcohol concentration” and “blood alcohol level.” Likewise, the term “breath alcohol content” is also referred to “breath alcohol concentration” and “breath alcohol level.” All of these terms are referred to in abbreviated form as “BAC.” 

The amount of alcohol in a person’s bloodstream is measured in milligrams (mg) of alcohol per 100 milliliters (ml) of blood. The number is typically expressed as a percentage (e.g,  0.08, 0.15). If a person has a BAC of .10 percent, that individual’s blood supply contains one part alcohol for every 1,000 parts blood. Blood alcohol concentration levels are determined through blood testing.

Impairment can also be measured through breath alcohol concentration. Breath alcohol concentration is an indirect (approximate) measure of blood alcohol concentration. Breath alcohol concentration is measured through a breath analyzer, colloquially referred to as a “breathalyzer.” 

In a breathalyzer test, the user exhales into the breath analyzer. A series of chemical reactions results in the breath ethanol (ethanol is the intoxicating agent found in alcoholic beverages) being converted to an electric current. The amount of current produced is then displayed on the breath analyzer device as the person’s breath alcohol concentration, also expressed as a percentage.

What is Driving While Impaired (DWI)?

All 50 states have established what is known as a blood alcohol concentration (BAC) limit. In most states, that limit is set at .08%. If an individual’s blood alcohol equals or exceeds that limit (typically a BAC of 0.08% or higher), the law presumes (regards) the person is driving while impaired (DWI). 

To put it simply, if an individual who is operating or controlling a motor vehicle takes a BAC test that reveals a BAC of .08% or higher, then the person, by definition, is driving while impaired (sometimes referred to as driving while intoxicated). The offense of driving while impaired or intoxicated is regarded as a “per se” (in and of itself) offense – a number of .08 or higher is by itself enough evidence for a conviction. 

Therefore, it does not matter if the person with the .08 level shows physical or mental signs of impairment, or, for that matter, if the person is operating the vehicle with perfect safety. So no matter what, .08 = driving while impaired. 

As respects the crime of DWI, there is not a “correlation” as such between BAC and impairment. A certain BAC (.08) is regarded as – equals – impairment, by definition.

At the same time, a person’s BAC at any given point in time is influenced by a number of variables. How quickly a person’s BAC rises after consuming a given amount of alcohol depends upon the following factors. These include (among other factors): 

  • Age
  • Gender
  • Weight
  • Body Fat Percentage   

What is Driving Under the Influence (DUI)?

Driving “under the influence” (DUI), like DWI, is a criminal offense. Whether a person is driving under the influence is not specifically determined by whether their BAC is at or above a certain percentage. Rather, a person is “driving under the influence” if:

  • Their ability to drive safely is diminished; and
  • The diminishment is the direct result of the person’s being in a state of intoxication.

DUI is a “cause and effect” crime. For the crime to be proven, there must be a particular result (diminished driving ability) brought about by a particular cause (intoxication). The cause and effect must both be proven.

BAC comes into play here; higher BAC levels are associated with higher levels of diminished or impaired driving ability. The more drinks one consumes, the higher the BAC becomes. The higher the BAC, the greater the degree of diminished driving ability or impairment. 

The National Highway Traffic Safety Administration describes the positive correlation between BAC and impairment in specific detail: 

A BAC of .02 has the following effects on driving: decline in visual function and in the ability to perform two tasks at the same time.

A BAC of .08 has the effects associated with .02, and is also associated with short-term memory loss; inability to control driving speed; reduced information processing capability (e.g., signal detection); and impaired perception.

A BAC of .10 has the effects associated with .02 and .08, as well as the effect of reducing one’s ability to maintain lane position and brake appropriately.

The more one’s ability to drive becomes diminished – the higher the BAC is – the more likely it becomes that an individual has committed the offense of driving while under the influence.  

Do I Need a DUI/DWI Lawyer?

If you are arrested for a DUI or DWI, you should speak to a DUI/DWI lawyer. This lawyer can advise you as to your rights, and can explain the differences between DUI and DWI. The lawyer may also represent you in hearings (including plea hearings) and at trial.