Driving Under The Influence of Alcohol
Physiology of Driving Intoxicated

The Physiology of Intoxicated Driving: Vision, Motor Skills & Reaction Time

Updated Dec. 25, 2020

Driving is an activity that requires physical coordination, swift reactions, keen observation and the ability to process information effectively. As alcohol intoxication has a detrimental effect on each of these skills, it will significantly impair your ability to drive safely.

For instance, if you drive while intoxicated:

  • Impaired vision may leave you unable to interpret road signs or spot hazards on the roadway.
  • Impaired reactions will make your maneuvers sloppy. You may turn widely, stop too abruptly or not abruptly enough.
  • Impaired focus may mean you forget important tasks, such as switching on your headlights or checking your speed.

In addition to these obvious impairments, the fact that you are intoxicated may endanger you if you begin to feel unwell. Alcohol can cause lightheadedness, nausea and a whole host of other physical complaints that may distract you from the driving task. Let’s analyze the specific dangers of driving under the influence in more detail.

Alcohol’s effects on vision

It is impossible to drive safely without good vision. While other senses are important, most of the information a driver gathers about the roadway environment is collected via the eyes. When vision is disrupted, your mental picture of the road around your vehicle will be incomplete. If vital information is missing (for instance, the presence of a stop sign, or approaching cross-traffic) your evaluation of the roadway will be inaccurate, and you will not be able to make safe driving decisions.

Object fixation

Object fixation (staring at one object or point on the roadway for too long) is a serious risk for intoxicated drivers. As alcohol affects the nervous system and limits your brain’s control over your body, the tiny muscles which move your eyes in their sockets are affected. When your eyes cannot move so easily, you will be more susceptible to fixating on a single spot. While this happens, you may overlook all sorts of other important events occurring elsewhere on the road. Objects to the rear or side of your vehicle and anything occupying your blind spots will likely go unnoticed.

Inability to focus

The corneas and lenses in your eyes become less responsive to signals from the brain when you have consumed alcohol. These components usually allow your eyes to focus on a specific object, to gather rich, detailed information about a certain part of the road. While you are intoxicated, you will struggle to focus on fine details like road signs, traffic signals or scenes on the roadway at a distance. If alcohol has impaired the precision of your eye’s lenses too, you may even start to see double.

Blurred vision

While under the influence of alcohol, your long-distance vision will become poorer. Objects, other vehicles and vital traffic control devices in your target area up ahead will appear blurry. You could have trouble distinguishing between lanes on the road, or pin-pointing the precise location of other road users.

Light sensitivity

Alcohol affects your ability to identify contrast. This will lead to difficulty distinguishing between different objects on a patch of roadway that is dimly (or very brightly) lit. When you search the roadway ahead, you may confuse a pedestrian with a street sign, or another vehicle with a dumpster. This inability to distinguish between objects is a danger in any conditions, though it is particularly lethal at night, in bad weather or when visibility is already poor for some other reason. Tragically, most DUI incidents do occur during the night when visibility is poor, as social drinking occasions generally happen in the evening. It is no wonder that drunk drivers account for so many night-time collisions.

The inability to see contrast is a result of alcohol’s effect on the eye’s pupils. In a healthy, sober person, the pupils rapidly expand and contract to allow more light in or to prevent dangerously bright light from entering, based on changing light conditions. This reaction is significantly slowed by alcohol. While intoxicated, you may find that:

  • A sudden shift from dark to light (as a vehicle with active high beams approaches at night) could cause pain and temporarily blind you.
  • A sudden shift from light to dark (as you turn from a well-lit street into a darkened side road) could leave you unable to pick out any details at all. Everything would appear black until your eyes can adjust, by which point you could have collided with another road user or driven straight off the road.

Depth perception and peripheral vision

Drivers often experience “tunnel vision” when they have been drinking, as alcohol reduces the scope of peripheral vision. This would leave you significantly less aware of events to the side of your vehicle. Furthermore, your depth perception will be affected. When depth perception is impaired, you will have difficulty determining how far away objects are and how much space is around your vehicle. As a result, it will be practically impossible to manage space on the roadway effectively.

Vision is arguably a driver’s most important skill. Without it, all other actions you take and the decisions you make behind the wheel will be misinformed and potentially unsafe. Do not let alcohol-fueled poor judgment convince you that you’re a “good enough” driver to drive safely when you’ve had a drink. There is no driver on the planet good enough to drive safely without full use of their eyes!

Alcohol’s effects on motor skills

We have touched upon alcohol’s effect on motor skills in previous modules of this block. Now, we’re going to explore the issue in greater depth to find out what impaired motor skills really mean for you as a driver. The term “motor skills” describes a person’s ability to move their body. A person with good motor skills has:

  • The ability to move their body accurately (e.g. reaching out to press a small button on a board of controls without “missing” it)
  • The ability to control the speed of movement (e.g. shooting your hand out to snatch an object before it falls to the ground, or moving your hand slowly and smoothly over paper to draw a delicate line)
  • The ability to control the force of movement (e.g. to grip a heavy object firmly or a delicate object lightly)
  • The ability to coordinate more than one movement (e.g. pressing the brake pedal with your foot while shifting gears with your hand).

These motor skills are absolutely vital to the physical act of driving. Since learning to control our bodies in early infancy, we do each of these things thousands of times a day without giving it a second thought.  While under the influence of alcohol, however, the messages sent from our brains to control our motor skills become disrupted and we can no longer move our bodies accurately, quickly, with precise force or in a coordinated manner.

Alcohol’s effect on motor skills is the reason a police officer may ask a suspected drunk driver to walk in a straight line or touch the end of their nose with their eyes shut. If they are even slightly impaired by alcohol, they will not be able to execute these simple tasks. Now, imagine how your driving skills will be affected if you cannot even walk in a straight line! Driving is a vastly more complex physical task.

While driving under the influence of alcohol:

  1. 1

    Poor muscle control will make it hard to execute basic driving tasks, like shifting gears, activating your turn indicators or applying gradual pressure to the brake.

  2. 2

    Lack of balance and poor control over speed and force will lead you to overcompensate for minor mistakes.

  3. 3

    Poor coordination will result in inefficient movement and trouble managing multiple tasks in succession.
    Maneuvers that require several actions at once (such as pulling off from a standstill or executing a turn) will seem incredibly difficult.

  4. 4

    The inability to move quickly will slow your reaction time.
    You may decide to maneuver around a hazard in the road but be physically incapable of moving fast enough to avoid a collision!

Diminished reaction time is probably the most dangerous effect of alcohol consumption, besides vision impairment. Avoiding an immediate danger is not the only time you must react quickly behind the wheel. Ordinary driving events like stopping at an intersection or merging into a new lane will become immediately dangerous if you cannot execute movements quickly.

Reaction time

Driving while under the influence of alcohol is a lethal mistake. Attempting to drive with either poor vision OR limited motor skills would be incredibly dangerous, let alone when these impairments occur side-by-side! If you jump into the driver’s seat with alcohol in your system, you will have to contend with both these handicaps, plus weakened memory, the inability to think clearly and poor judgment.

While alcohol’s negative physical and mental effects are far-reaching and complex, they can all be boiled down to one crippling impairment: slowed reaction time. Hopping into the car and driving from A to B without incident depends on your ability to adapt your behavior as the roadway environment changes. When you are intoxicated, EVERY stage of this process is disrupted:

  1. 1

    You may not SEE the need to change your behavior due to poor vision.

  2. 2

    You may not RECOGNIZE the need to change your behavior if your memory and reasoning are impaired.

  3. 3

    You may CHOOSE an inappropriate response to the changing environment if your judgment is impaired.

  4. 4

    You may not be able to EXECUTE a change in behavior effectively if your motor skills are impaired.

Even if your intoxicated mind and body manage to work through each of these stages without making a mistake, it will do so far slower than usual. By the time you attempt to stop, slow down, speed up, merge or turn, you could have run a red light, collided with another car, hit a pedestrian or plowed off the side of the road.

No safe level of intoxication

What is a safe amount of alcohol to drink before driving? This is a very important question, with a very clear answer.

The answer is: there is NO safe amount.

People assume that any amount of alcohol that leaves them under the legal BAC limit of 0.08% must be safe, because it is legal. This could not be further from the truth. We know that two people with the same blood alcohol concentration can experience totally different levels of intoxication. Plus, impairment from alcohol intoxication starts just minutes after the first sip of your first drink. There is research to suggest that alcohol affects a person’s behavior even before it reaches their brain!

When under the influence of any amount of alcohol, people are known to behave differently and make different choices. Depending on the situation, you may become more aggressive, reckless, spontaneous or sloppy. Even if your vision and motor skills appear to be unaffected, driving while experiencing these behavioral changes would be an extremely bad idea.

More than anything, alcohol blinds a person to how intoxicated they are. What feels like mild intoxication to you might look like extreme drunkenness to sober people around you. Partner this effect with bad judgment and the chances of you deciding to drive while impaired by alcohol are high.

The stages of intoxication

Everyone responds to alcohol differently, though there are certain mental and physical effects that can be generally associated with different levels of intoxication. Here, we detail the different states of alcohol intoxication and their effects, using blood alcohol concentration as a measure of intoxication. Keep in mind that the amount of alcohol a person has consumed does not necessarily determine their BAC. How much alcohol makes it into your blood and how you respond to it is determined by your age, weight, emotional state, experience with alcohol, food intake and genetics.

That being said, it can be useful to have a rough idea of how your body will react and change as you drink. Let’s check out a typical alcohol consumption timeline.

The first drink (0.02% BAC)

You may think that you’re safe to drive after just one drink, but studies have demonstrated that a BAC of just 0.02% can adversely affect driving ability. Shortly after starting your first drink, you will feel more relaxed and have a harder time concentrating.

BAC 0.02%  

Most people with a BAC of 0.02% experience:

  • Mildly impaired vision, including limited ability to locate and track objects
  • A change in mood
  • Mild reasoning and judgment impairment
  • Difficulty focusing their attention on a task


The “increased risk” threshold (0.05% BAC)

When your BAC hits 0.05%, you will begin to notice the effects of alcohol. At this level, most people find they feel very relaxed, euphoric and more sociable than usual.

BAC 0.05%  

The physical and mental effects of alcohol at this level of intoxication are:

  • A further decline in reasoning ability, judgment and alertness
  • Impaired physical coordination
  • Increased reaction time
  • A further decline in vision, including difficulty focusing and weakened light sensitivity
  • Dulled senses
  • Exaggerated behavior

As you can see, your ability to drive safely can be significantly impaired even before you reach the legal BAC limit.

The legal limit (0.08% BAC)

In most states, a BAC of 0.08% or higher is over the legal driving limit for drivers over 21 years old. Underage drivers and commercial drivers are generally subject to a lower limit. At 0.08% BAC, most people become very talkative and excited. In addition, you will have difficulty focusing and processing information. Physical tasks will also be harder to execute effectively.

BAC 0.08%  

At this stage of intoxication, you can expect:

  • Trouble performing several actions in succession
  • Significantly reduced reaction time
  • Disconnection from your surroundings, often resulting in poor speed control while driving
  • Overconfidence and a tendency toward high-risk behavior
  • Significant impairments in focus, memory and judgment
  • Poor balance, slurred speech and sensory impairments
  • Significantly impaired vision


Over the limit (0.10% BAC)

Beyond the legal limit of 0.08%, the physical and mental effects of alcohol will become considerably more apparent. People with a BAC of 0.10% experience very bad coordination, slurred speech and complete loss of inhibitions. Frighteningly, at this stage, many people still do not realize they are intoxicated.

BAC 0.10%  

With a BAC of 0.10%, you will suffer with:

  • Dangerously increased reaction time
  • Severe focus and judgment impairment
  • Significant trouble controlling your vehicle, including inability to stay in your lane and difficulty braking properly


The high danger zone (0.15% BAC)

Vision, judgment, focus and motor skill impairment will continue to worsen as your BAC increases.

BAC 0.15%  

At 0.15%, you will be completely incapable of controlling your vehicle or thinking clearly, due to:

  • Complete or near-complete loss of muscle control
  • Nausea, vomiting and a sudden drop in body temperature
  • A potentially fatal slowing of heart rate and breathing
  • Extreme confusion
  • Complete or near-complete detachment from your immediate surroundings
  • Failure to recognize people, situations and objects


Beyond 0.15% BAC

Blood alcohol concentrations above 0.15% are extremely dangerous and often deadly. People with BACs beyond this level are usually completely incapable of engaging with the world around them in any meaningful of sensical way. Here’s what would happen if your BAC continued to climb.

BAC 0.20% and Higher  

  • At 0.20% BAC you will be overcome with drowsiness and incapable of controlling your body.
  • At 0.30% BAC you will become unconscious and are at risk of falling into a coma.
  • At 0.40% you will reach a life-threatening level of suppressed respiration and irregular heartbeat.
  • At 0.50% your breathing and heartbeat will be so disrupted that death from alcohol poisoning is likely.

Contrary to what many people tragically assume, it is not particularly difficult to hit a BAC of 0.40% or 0.50% if you are not drinking responsibly. For a man of 150lbs, it would take 12 standard drinks consumed within a two-hour period to reach a BAC of 0.50%. This is easily done and has happened many times as a result of wild parties and peer pressure.

Fatal collisions due to alcohol impairment

Any level of alcohol intoxication increases a driver’s chances of being involved in a fatal crash. However, a person driving with a BAC of 0.08% or higher (over the legal limit) faces a significantly greater risk of death in a traffic collision.

Fatal Alcohol-impaired Crashes  

In 2017, 29% of all fatal collisions in the United States involved at least one drunk driver. Of these fatal alcohol-impaired crashes:

  • 17% involved drivers with a BAC of 0.01 to 0.07%
  • 25% involved drivers with a BAC of 0.08% to 0.14%
  • 58% involved drivers with a BAC of 0.15% or higher

The fact that nearly 60% of fatal drunk driving crashes involved drivers who were significantly over the limit demonstrates alcohol’s profound effect on judgment. Despite the obvious illegality of the act and the noticeable impairments these people would have been suffering with, they still thought driving was a good idea.

Alcohol-impaired driving among teenagers

Young drivers are the most at risk of causing fatal drunk-driving collisions. One study found that drivers aged 21 to 24 are the most likely to kill themselves or another person by driving while drunk. If it wasn’t for the under 21 “zero tolerance” policy which prohibits underage people driving with any alcohol in their system, it is likely to be drivers under 21 who cause the most deaths. Teenagers and young people are particularly affected by poor judgment as a result of alcohol consumption.

Another study focusing on drivers between the ages of 16 and 19 examined how different BAC levels influenced a driver’s risk of dying in a single-vehicle collision. The results showed that, as compared to a BAC below 0.01%:

  • a BAC of 0.015% to 0.049% increased the driver’s chances of dying 2.5 times.
  • a BAC of 0.05% to 0.079% increased the driver’s chances of dying 9 times.
  • a BAC of 0.08% to 0.099% increased a driver’s chance of dying 40 times.
  • a BAC of 0.10% to 0.149% increased a driver’s chance of dying 90 times.
  • a BAC of 0.15% or above increased a driver’s chances of dying 420 times.

These results demonstrate that even a slight bump in BAC leads to a substantial increase in the risk of death in a single-vehicle collision for teenage drivers. Before you even hit the legal BAC limit for adults, you will have a 9 times greater chance of making a fatal mistake.

The DUI death toll

Around 10,000 people die in alcohol-impaired traffic collisions every year across the United States. Such an enormous loss of life is difficult to comprehend as drunk-driving accidents are entirely preventable. Drivers must realize that operating a vehicle with any amount of alcohol in their system is unsafe and leaves them at greater risk of making a mistake. The physiological effects of alcohol are profound, and they begin to take hold the very moment an alcoholic drink touches your lips. A person who has consumed alcohol is immediately less capable of scanning the roadway, making safe decisions and executing maneuvers effectively.

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