Driving Under the Influence of Alcohol: DUI Rules and Regulations

Alcohol has a very specific effect on physical and physiological functions, which are vital to the ability to operate a motor vehicle. As a depressant, alcohol mainly affects the function of the brain. It suppresses vital brain centers, such as those in the prefrontal cortex responsible for the ability to shift attention from one thing to another. Drivers under the influence have a more limited area of usable vision than their sober counterparts on the road. The information flow from the brain to the eyes is disrupted when the eyes must move quickly from one stimulus to another or turn sideways to detect stimuli.

Driving under the influence is legally defined as operating a motor vehicle when the alcohol content in the blood exceeds the legal limit. The legal limit is seen as the level, beyond which a person isn’t able to drive safely. This level is 0.08% in most US states, but the laws can vary. Driving on private property is a no-defense. The blood content level is generally determined by a measurement of blood alcohol content (BAC) or a BrAC, a breath test measurement. A level exceeding 0.08% is a criminal offense without having to prove impairment.


DWI stands for driving while intoxicated or driving while impaired. Both DWI and DUI mean that a driver is being accused or charged with a traffic violation. Depending on the state, both terms are used to describe drunk or impaired driving. Some state laws use “DUI” to refer to the offense of drunk driving, and other state laws use DWI.

In some cases, a state can use both terms. One refers to driving after having consumed alcohol and the other denotes impairment by another drug. DWI can refer to driving under the influence of alcohol with a BAC that exceeds the legal limit, while DUI is used when the driver is operating a vehicle after consuming alcohol or taking another drug/ other drugs.

In other states that use both terms, DUI can mean driving under the influence of alcohol, while DWI means driving while impaired by alcohol, drugs, or an unknown substance. Legal definitions vary by state.

Additionally, DWI can mean the arresting officer has reason to suspect the driver cannot continue driving due to severe impairment. A driver faces this charge in some jurisdictions even if their BAC is below the legal intoxication level, which is usually 0.08%, as noted above.

Prevalence of Drunk Driving

How Does Alcohol Impair Driving Skills?

Alcohol impairs driving skills in several ways. These include aspects such as tracking, visual acuity, judgment, reaction time, and concentration. We’ll look at each of them briefly below.

Alcohol can compromise a driver’s ability to judge their position on the road, the position of road signs, the location of the centerline, and the position of other vehicles. The likelihood of a serious accident caused by driving under the influence is quite high as a result. A driver’s ability to control and maintain the position of the vehicle relative to changes in the surroundings is diminished. Tracking errors can contribute to both head-on collisions and run-off-road single vehicle crashes. A NHTSA study by Herbert Moskowitz and Dary Fiorentino found consistent impairment at a BAC level of 0.005.

Alcohol can slow down a driver’s reflexes and decrease their ability to grasp and react to changing situations swiftly. Studies by the Centers for Disease Control (CDC) show that drivers under the influence of alcohol find it impossible to respond to stimuli quickly enough. Reaction time due to impaired coordination and comprehension may slow down by up to 25 percent. Slower reactions may cause collisions and accidents leading to injuries and possibly fatal outcomes.

When one is under the influence of alcohol, their visual acuity is reduced by up to 32% according to a study carried out by the University of Texas in San Antonio. This impairment manifests in reduced peripheral vision, causing tunnel vision. The ability to judge distance and depth accurately is diminished. Pupils become dilated, which results in poorer perception of stimuli such as the glare of oncoming headlights. Hearing is impaired as well – persons driving under the influence have a harder time determining the direction, from which sounds come.

Alcohol use can affect the ability to understand or accurately interpret signals, road signs, and situations, in which a driver must respond quickly in order to ensure road safety. Those driving under the influence become easily confused and thus cannot respond adequately in an emergency situation. The ability to think clearly and rationally, make sound decisions, and plan ahead is more limited, even with blood alcohol content levels as low as 0.02%.

A study by the NHTSA titled “Effects of Low Doses of Alcohol on Driving-Related Skills“ showed a strong inverse correlation between alcohol use and concentration levels. More specifically, the ability to concentrate on more than one task simultaneously and to make rational decisions was compromised. The study suggested impairment can start at BAC of just 0.005. Many traffic accidents are caused by drunk drivers who have a hard time focusing or are easily distracted.

How Does BAC Affect Driving?

The CDC divides BAC into five levels, ranging from 0.02 to 0.15, with 0.08 being the limit in almost all states. Below is a brief description of each.

  • poor judgment
  • increased relaxation
  • mood swings
  • slightly increased body
  • temperature
  • inability to multi-task
  • limited visual functionality
  • lack of judgment
  • lack of coordination
  • exaggerated behavior
  • lack of alertness
  • diminished ability to detect
  • moving objects
  • reduced response rate
  • low inhibition
  • limited small-muscle control


  • reduced muscle coordination
  • lack of reasoning
  • loss of short-term memory
  • lack of self-control and
  • speed control
  • reduced ability to
  • concentrate
  • very limited ability to process information

Fatal crashes involving BAC of 0.08 percent or more are charged as alcohol-impaired driving fatalities.


  • poor coordination
  • limited ability to control the vehicle
  • slurred speech
  • reduced ability to brake when needed
  • inability to keep the vehicle within the lane fatalities.
  • no muscle control
  • extreme loss of balance
  • greatly diminished sensory information processing
  • very poor ability to turn attention to driving

Notwithstanding the fact that alcohol-impaired driving fatality rates have dropped by 27 percent between 2005 and 2014, the US government (more specifically US taxpayers) loses $44 billion a year from motor vehicle accidents involving alcohol.

Federal Drunk-Driving Laws

Most DUI offenses are prosecuted by state, not federal governments, because state courts have jurisdiction over DUIs and other violations of state laws. The opposite is true for violations of federal laws. Federal DUI charges are reserved for crimes that take place on federal property, such as US military bases, national monuments or historic grounds, airports, courthouses and surrounding property, any US-government-owned building or parking lot, and national parks and forests.

The most common DUI incidence is military men drinking on federal property or federally-owned roadways. After being pulled over and taking a field sobriety test, one can be charged with a federal DUI if their BAC is over 0.08. Members of the military may face further repercussions for DUI according to the Uniform Code of Military Justice. Most first-time DUI offenses are considered misdemeanors under federal law, punishable by incarceration in a county jail for 12 months or less or a fine in lieu of jail time.

According to federal legislation, the statutory limit for commercial drivers is 0.04. A commercial driver with an alcohol concentration between 0.02 and 0.04 is removed from duty for a period of 24 hours.

State Drunk-Driving Laws

In some states, it is illegal to drive a motor vehicle while intoxicated or under the influence, while it is also illegal to operate a motor vehicle in others. There is a difference in the legal terminology, i.e. “operation and control” and “driving”. “Driving” is taken to mean movement of the vehicle in some direction, while “operate” includes not only movement, but also acts which may cause the vehicle to be set in motion. According to this definition, sitting drunk in the car seat with the keys in your hand, not in the ignition, is a punishable offense.

In addition, each state designates a blood alcohol level as the legal limit, a threshold point for a criminal offense. Refusing to take a test carries additional penalties. Refusing to a blood or breath test in California, for example, carries a one-year license suspension as per California Vehicle Code Section 13558(c)(1).

BAC levels at which different types of drivers are considered legally drunk

StatesNon-commercial driver over 21Commercial driverDriver under 21
Alabama, Alaska, Arkansas, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Mexico, North Carolina, North Dakota, Ohio, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Vermont, Virginia, West Virginia, Wyoming0.080.040.02
California, New Jersey0.080.040.01
Illinois, Maine, Oklahoma, Oregon, Texas, Washington, Wisconsin0.080.04Anything above zero
Michigan 0.08 (up to October 1, 2018)

0.10 or more (as of October 1, 2018)

0.04 0.02
New York0.080.04Drivers under 21 are legally drunk after a chemical test shows they used alcohol
Utah0.08(up to December 30, 2018)

0.05 (as of December 30, 2018)


Below is a short list of the penalties for drunk driving per state. This data refers to first-time offenses.

DUI Arrest Rates per State

Based on the number of arrests and population, the states with the highest number of DUI arrests per capita are Wyoming (1.17%), Idaho (0.78%), Alaska (0.75%), Nebraska (0.71%), and South Dakota (0.70%). Conversely, the states with the lowest number of DUI arrests per capita are Washington D.C. (0.01%), Delaware (0.03%), Illinois (0.03%), Ohio (0.16%), and New York (0.19%). Data is based on sources of the University at Albany and the US Census Bureau.

What are the reasons behind these statistics? You can see that the states with the lowest rates are in close geographic proximity, some even neighboring one another. On the other hand, states with high rates are more sparsely populated, lending a sense of freedom and impunity to drivers. These states also have less severe penalties for DUI – up to 3 months in prison and fines of less than $1,000 in most, while states with low rates have penalties of at least 6 months in jail and fines of over $1,500.

Investigation and Arrest Procedure

A DUI charge is a police arrest procedure involving reasonable suspicion, probable cause, arrest, and criminal charge with civil fines. Civil fines are not intended as punishment for a crime, but rather as a means to compensate the state for damage done to it. Arrest invokes the implied consent law.


According to the NHTSA, the investigation stage is distinct from the legal stages listed above. The first two goals of the investigation are to identify impaired drivers and establish clear probable cause to proceed to an arrest. Probable cause is required to sustain the use of chemical test results in a court of law. If probable cause is not established, tests results are not admitted as evidence in court. After it is established, the officer proceeds to obtain proof of intoxication, usually by means of a blood test.

Based on NHTSA recommendations, the stages of a DUI investigation are as follows:

  1. Moving Vehicle
  2. Personal Contact
  3. Screening before Arrest

The first stage involves observations by law enforcement officers of the driver’s maneuvers. The officer comes into personal contact with the DUI suspect in the next stage. In the final stage, the officer questions the suspect prior to administering to them Field Sobriety Testing, of which preliminary alcohol screening may be part. This stage also includes an evidentiary chemical test.

As the officer approaches the suspect’s vehicle to ask questions, he or she watches out for signs of intoxication, such as the smell of alcohol on the driver’s breath or in the car in general, bloodshot or watery eyes, slurred speech when responding to the questions, droopy eyelids, fumbling with the license and registration documents, alcoholic beverages in plain sight in the vehicle, difficulty in perception and response to questions, and admitting to drinking.

Valid Reasons and Exclusionary Rule

An officer must have a valid reason to pull a vehicle over. This is a basis for further prosecutorial action. Pulling someone over would be illegal without such basis. The suspect cannot be prosecuted because the evidence would be considered to have been obtained without his consent under the exclusionary rule, which is grounded in the Fourth Amendment of the US Constitution and intended to prevent illegal searches and seizures.

Valid reasons for a traffic stop include observation of a traffic violation behavior, such as leaving the lane or weaving, which would invoke a “reasonable suspicion” of driving under the influence. These are the most common reasons for DUI stops. Other valid reasons would be if a police officer notices signs of intoxication after stopping a vehicle for a lesser traffic offense, if the driver was in an accident and the officer arrived on the scene to investigate, the police have received an erratic driving report (erratic driving must be verified, especially if the report was anonymous), or the driver was stopped at a roadblock or sobriety checkpoint.

The NHTSA’s Guide for Detecting Drunk Drivers, used to train police officers, lists the following symptoms of DUI:

All these are valid reasons for a stop.

Reasonable Suspicion

If the police officer finds sufficient proof of reasonable suspicion, which provides legal grounds for further investigation and detention, he or she will ask the driver to step out of the vehicle. During the stop, the police will try to obtain evidence of probable cause, which is a prerequisite for arrest. This is possible through observation or, more easily, confession.

If the driver admits to having consumed alcohol, probable cause is practically a given. The police will ask the suspect if they have been drinking, but they won’t read them their Miranda rights because they are not being accused of anything (yet) and because the police want to lure them into believing the questions are being asked for a reason other than obtaining probable cause for arrest. At this point, the driver is not obligated to provide more than his license and registration from a legal standpoint.

Testing Procedure

Breath test or field sobriety test results can also yield probable cause. These tests are not mandatory by law and the driver can choose whether to take them.

Likewise, the police are not obligated to communicate to the suspect that the tests are voluntary.

Field Sobriety Tests

The NHTSA has three validated field sobriety tests: the One-Leg-Stand Test, the Walk-and-Turn Test, and the Horizontal Gaze Nystagmus Test. The horizontal gaze test involves tracing an object visually to establish typical eye movement. For the walk and turn test, you have to walk in a straight line heel-to-toe. This test is aimed at assessing a person’s ability to remember a series of steps while alternating attention between mental and physical tasks. According to the NHTSA, these tests were designed to assess the likelihood of a driver’s BAC being at or above 0.08%. Alternative, non-validated tests include the Alphabet Test, the Counting Test, Romberg Test, the Finger Count Test, and the Finger-to-Nose Test.

Preliminary Alcohol Screening Tests

Preliminary alcohol screening tests, also referred to as preliminary breath tests, are increasingly used as a way of obtaining probable cause for arrest. These tests precede the actual arrest and ensuing requirement to the suspect to undergo evidentiary chemical testing of the blood or breath. Preliminary tests are carried out using small, inexpensive breath testing devices. There are larger, more complex versions of these devices at police stations, commonly referred to as Evidentiary Breath Test (EBT) devices.

More and more US jurisdictions are making use of portable EBT devices, which are more sophisticated versions of the preliminary test devices. Larger EBT devices have integrated infrared spectroscopy technology, whereas preliminary and portable devices use simple fuel cell technology.


After sufficient probable cause has been obtained, the officer will make an arrest. The suspect is handcuffed and transported to a police station. At this time, they are informed of their obligation to take a chemical breath, blood, or urine test to determine BAC according to the element of implied consent in driver licensing laws. Refusing to take such a test may result in punishment under the implied content principle of the law, including license revocation.

The explanation of the suspect’s rights by the arresting officer generally includes a description of the repercussions of refusing to take a chemical test and being informed of the right to obtain a urine or blood sample so the suspect can conduct a test on his own.

If the suspect was not informed of the ramifications of refusal, the test result might not be admitted by the prosecution as evidence in their case and they could challenge license revocation.

Test results are usually available immediately. Blood and urine samples are sent to a lab to establish the BAC or possible drug use. Chemical tests, however, do not establish the level of impairment of the driver. A BAC of 0.08% or higher is legal grounds for rebuttable presumption of intoxication in all states.

Criminal Charges

A suspect whose BAC is below the legal limit, which is 0.08% in most states, will be released without any charges in the vast majority of cases. A DUI charge would still be possible based on symptoms such as observed field sobriety test performance, impairment, or admissions of the individual. Most of the time, the individual is held in a cell until the authorities find they have sobered up to a sufficient degree to merit release on bail.

Alternatively, the arrestee can be released on his “own recognizance”. OR is where they acknowledge their debt to state authorities. This acknowledgement (recognition) is avoided if the person keeps the peace or appears in court on a particular day.


The person is given a date, on which to appear in court for an arraignment. This is a formal reading of criminal charges in the defendant’s presence to inform them of the charges they face. The accused must then enter a plea. Pleas usually include “not guilty”, “guilty” (achieved through plea bargain), and pleas in bar or peremptory pleas giving reasons why a trial cannot proceed. If the arrestee cannot make bail or is not granted release on own recognizance, they will remain in custody on remand pending arraignment.


Also known as provisional or pre-trial detention, remand is the process of detaining an individual who has, in this case, been arrested and charged with DUI. As a person is presumed innocent until proven guilty, it would be illegal to detain them at this point, so suspects are remanded in relatively few cases. Grounds for remanding include a high likelihood of them committing a heavy crime, failing to appear on the arraignment date, or interfering with the investigation. They will be held in a remand or regular prison. Suspects are out on bail in the vast majority of court cases.

DUI Prevention Strategies

The strategies listed in this section have been found effective by the Guide to Community Preventive Services of the CDC in terms of reducing the incidences of drunk driving. NHTSA reviews have shown they can be helpful in preventing drunk driving. They go beyond drunk driving laws regarding BAC, which are in place in all US states and have saved tens of thousands of lives since being adopted.

More Arrests

Data of a study by Olson and Gerstein published in the National Academies Press showed an estimated 500 to 2,000 DUI incidents went unpunished for every arrest made, although more arrests are made for this reason than for any other offense in the US. To counteract this tendency, the Department of Transportation funded a series of locally managed and organized alcohol safety projects in different states, aimed at combining a higher risk of arrest, public education, and more effective trial procedures and rehabilitation. By motivating law enforcement to be more active, targeting patrols at specific places and times, and enhancing surveillance, the number of DUI arrests doubled and even tripled in most states.

Around half of the projects also had a discernible effect on road fatalities at night. More specifically, these decreased by 30% on average over three years. The National Research Council concluded that a higher risk of arrest does deter driving under the influence.

Most Effective Road Safety Measures

Most alcohol-related and induced traffic fatalities happen at night, which is why increased surveillance is particularly important then. What is more, studies have shown that the speed of court procedures can influence the effectiveness of drunk driving laws substantially. Drunk driving is a prime example of both the possibility and need of complex prevention measures. Obviously, strict drunk driving laws enforced by the police and held up by courts of law play a key role, but legal action alone is not sufficient to resolve this issue and guarantee road safety. Educational strategies in combination with statutory measures will help achieve the best possible outcome.