Idaho Traffic Safety Resource Prosecutor

Driving Under the Influence

Online Edition of the Idaho DUI Factbook

This online edition of the Idaho DUI Factbook is as accurate as possible at the time of publication. This manual does not cover every aspect of Idaho's DUI laws and should not be cited as a legal authority in court.

“Driving Under the Influence” is defined as driving or being in actual physical control of a motor vehicle while impaired by alcohol, drugs or any other intoxicating substances. In Idaho, a driver is legally considered to be under the influence if the driver has a blood-alcohol concentration (BAC) of .08 percent or greater, has used any illegal substance, or is impaired by any legal drugs or intoxicating substances or any combination thereof. 

Historically, neither scientists nor legal scholars could agree on a definition of “impairment” or “under the influence.” In 1938, the National Safety Council’s Committee on Alcohol and Other Drugs (CAOD) collaborated with the American Medical Association’s Committee to Study Problems of Motor Vehicle Accidents to establish standards for defining the phrase “under the influence.” Based on the research at that time, these committees established presumptive levels, defined in terms of blood alcohol concentration. Based on their recommendations states began enacting statutes wherein the “presumptive levels” shifted focus from officer observations to chemical testing in impaired driving investigations. In 1971, CAOD recommended lowering the presumptive level to 0.08.  However, it took decades before this recommendation was reflected in criminal statutes.

When the Idaho Legislature passed legislation in 1997 to lower the illegal BAC limit to .08 percent from .10 percent, Idaho became the 14th state to impose such a change. Today, all 50 states, the District of Columbia and Puerto Rico have .08 BAC per se laws. BAC limits are lower for drivers under the age of 21 and drivers of commercial vehicles.

Blood Alcohol Limits in Idaho
Drivers Age 21 and Over 0.08
Drivers Under Age 21 0.02
Drivers of a Commercial Vehicle 0.04

Based on decades of research, there is scientific consensus that alcohol causes deterioration of driving skills beginning at 0.05 BAC or even lower. From the first drink, alcohol affects coordination and judgment. Even with a BAC well below .08 percent, a person's reaction time slows. The risk of being in a crash begins to increase between a BAC of .04 and .05 percent and increases rapidly thereafter. By the time a driver reaches a BAC of .06 percent, he/she is twice as likely to be involved in a fatal crash as a non-drinking driver.  By the time a driver reaches a BAC of .08 percent, he/she is 11 times more likely to be killed in a single-vehicle crash than a non-drinking driver.  The American Medical Association now supports a policy recommending lowering the illegal BAC limit to 0.05 percent.

The effect of alcohol on an individual is determined primarily by two factors: the amount of alcohol consumed and the rate at which it is absorbed by the body. Other contributing factors include gender, body weight, alcohol tolerance, mood, environment and the amount of food consumed.

The only way to rid the body of alcohol is the passage of time. Fresh air, coffee, a cold shower and food cannot help a person sober up. It takes approximately one hour for the body to metabolize one drink. Each of the following has a comparable amount of alcohol and counts as one drink: one 12-oz. mug of beer, one 5-oz. glass of wine or one 1.5-oz. shot of hard liquor. (The amount of alcohol in a poured/mixed drink depends on the type of drink and the person who pours it.)