Effects of Alcohol

The National Highway Traffic Safety Administration (NHTSA) in the US conducted a review of 109 research studies into the effects of alcohol and impairment.

The following information shows the Blood Alcohol Contents (BAC) at which impairment was first reported. In some instances this does vary between studies.

For detailed information see the original Effects of Low Doses of Alcohol on Driving-Related Skills report.

Research summary

Divided attention

Divided attention involves having to concentrate on two or more tasks at the same time and making decisions about them. Attention can be given to tasks occurring in front of us, in our peripheral vision, or even to information we are listening to.

For example: you’re driving down a suburban street, you are concentrating on maintaining your current position on the road to avoid hitting parked cars and any oncoming traffic. At the same time you are scanning the street numbers looking for your friend’s house. Your attention is divided between these two tasks.

Impairment level: the review of the research showed that dividing attention between tasks was very sensitive to alcohol consumption. In fact, some studies showed that subjects were impaired at a BAC as low as 0.005 suggesting that dividing attention between seemingly simple tasks as described can start to be affected at very low levels. If something was to unexpectedly occur in the driving environment which also required our attention, such as a pedestrian stepping onto the road, it will be more difficult to adequately shift our attention even at low BACs.


Vigilance is the level of alertness or watchfulness we are able to give to a task or an object in the driving environment.

For example: during peak hour, you’re driving in heavy traffic on the freeway. The flow of traffic can frequently change from moving at slow or medium speeds to stopping altogether. You need to be continuously watchful of the vehicle in front and the flow of the traffic up ahead to ensure you don’t hit the vehicle. Alternately, you’re driving through a strip shopping centre soon after school breaks up for the day. There are many pedestrians walking around cars parked on both sides of the road as well as pedestrians attempting to cross the road. You need to be continuously watchful and prepared in case a child or adult pedestrian unexpectedly enters the roadway.)

Impairment level: the review of the research indicated that vigilance can be impaired at BACs of at least 0.03.


Tracking refers to our ability to control and maintain position relative to changes in our driving environment.

For example: you’re driving along a winding country road, and you need to maintain your lane position between the centre line and the shoulder of the road without drifting outside of your lane. You need to do this despite the constant changes in direction of the road. Alternatively something might happen which constantly makes it difficult for you to maintain position such as sudden wind gusts as you cross over a long bridge. Attempting to maintain your lane position in both instances are examples of different forms of tracking whilst driving.

Impairment level: the review of the results showed that tracking is a skill very sensitive to alcohol, with impairment found at BACs as low as 0.0018 and consistently at 0.005 BAC. Tracking errors can be contributors to run-off-road single vehicle or head-on crashes.


Perception is a cognitive skill which refers to our ability to process and make sense of information presented to us whether that is through our vision, what we hear or what we feel. The ability to recognise potential traffic hazards is probably one of the most important aspects of perception and its relationship to the driving task and safety. The ability to estimate time and distances are also important.

For example: you’re driving down a local street in a country town and a ball rolls across the road up ahead. The ball in itself is not a hazard but balls and toys are usually associated with children. By making such an association and slowing down in preparation that a child may follow the ball onto the road is an example hazard perception whether the hazard ultimately presents itself or not.

Impairment level: the review of the research indicated findings of impairment at BAC levels as low as 0.04.


Visual functions include visual acuity (the ability to see fine detail in objects), contrast sensitivity, eye movements, and motor control of the eye.

For example: You’re driving at night, you can see the tail and brake lights of vehicles ahead of you and the headlights of oncoming vehicles. The size and brightness of those lights provide you with cues as to how far away the other vehicles are and how fast they may be travelling as indicated by changes in the size and brightness of the lights. Furthermore, you need to constantly shift your gaze and focus on the various aspects and objects of importance in your driving environment.

Impairment level: For some visual functions impairment started to become evident at BACs of about 0.03.

Psychomotor skills

Psychomotor skills include our ability to maintain balance and to perform and co-ordinate physical tasks.

For example: approaching an intersection to do a right hand turn, you need to change gear, brake, indicate, scan for oncoming traffic, make a decision on when it is safe to turn, steer the vehicle and perform the turn all in a co-ordinated sequence.

Impairment level: Balance can begin to become impaired at a BAC of 0.04. Complex tasks requiring co-ordination can be affected at 0.05.

Reaction time

Reaction time refers to the time between the occurrence of an event or appearance of an object and a persons reaction to it if one is required.

For example: You’re driving down a street when a car suddenly reverses out of a driveway in front of you. You have the choice of braking or swerving around the vehicle. Given that there could be oncoming traffic the safer option would be to brake, provided there are no vehicles behind you. The time it takes to assess the situation, make a decision and start to take action is your reaction time.

Impairment level:  the review indicated impairment of reaction time by alcohol at as low as 0.02 and consistently at a BAC of  0.06.


Unlike the other issues examined above, our degree of wakefulness is not a measure of our performance on a particular skill. Nevertheless, wakefulness is very important for safe driving and in very much influenced by alcohol intake. Drowsy driving due to sleep loss or deprivation is a major contributor to crashes. Considering that alcohol related crashes often occur at night when driver are more likely to be needing sleep, the combined effects of alcohol and drowsiness can lead to even greater impairment than either factor on its own.

Impairment level: At BACs of at least 0.01 the time taken to fall asleep is significantly less than if you didn’t consume alcohol. So the drowsier you are the more you are at risk of having a fatigue related crash even if you are within the legal limit.

Alcohol affects everyone differently

Alcohol is a drug. It acts on the brain and other parts of the nervous system to slow activity down. It affects everyone this way. This is why alcohol is such a problem for driving - it slows down reflexes and decision making and drivers don't always notice the effects.

The more you drink, the higher your blood alcohol level and the stronger the effect of the alcohol on your reflexes and decision making.

You and your friends will all have different physical reactions to the same amount of alcohol. Some people can drink more and still be under .05, and some get to .05 very quickly. This is because the same amount of alcohol can result in different blood alcohol levels for different people, and the same blood alcohol level has different effects on different people.

Stay in control with standard drinks

Standard drinks are a good guide to manage our alcohol consumption and therefore our likely BAC. A standard drink is one that contains 10 grams of alcohol. It enables us to monitor the level of our alcohol consumption. Different drinks are served in different sized glasses and containers often hold more than one standard drink. Counting standard drinks is the most reliable way we have of measuring how much alcohol we have consumed.

To stay below the 0.05 BAC limit, drivers are advised to limit their drinking to one standard drink per hour if you are a female and two standard drinks in the first hour, and one standard drink each hour after that if you are a male.

The only way to really know your blood alcohol level is to use a breath test device. The Police use these for random breath testing, but that is not the best way to find out you are just over .05. Many hotels, restaurants, and nightclubs now have breath testers.

Size does matter

Alcohol is absorbed into the bloodstream where it is transported around the body until the liver can metabolise it. Larger people can be affected less, or more slowly, than smaller people. In a smaller person alcohol is concentrated into a smaller area due to their smaller body mass, and so it has a greater effect.

For that reason, larger people usually have a lower blood alcohol level when they drink a particular amount of alcohol. Other factors are important, but this is why larger people seem to be able to drink more before showing signs of being intoxicated.

You can't easily predict how much more you can drink if you're a larger person, though, because this is only one of the many factors that can influence your alcohol level.

Gender also impacts on the effect of alcohol. Females are affected by alcohol more than males because their bodies carry more fatty tissue. As alcohol is not soluble in fat, it is concentrated into a smaller area in a female body and so has a bigger and quicker effect. Given the same amount of alcohol a female usually will have a higher blood alcohol level than a male.

Waiting a while makes you safer

Once alcohol is in your blood stream, your liver is responsible for removing it. There is nothing you can do to make this happen any faster. If your blood alcohol level is above .05 and you want to drive home, the only thing that will get your blood alcohol level below .05 is waiting for your liver to do its job. While you are waiting, of course, you shouldn't drink any more alcoholic drinks or your blood alcohol level will start to increase as the alcohol is absorbed.

It's difficult to know how long to wait. For most people, alcohol is removed by the liver at a rate of about one standard drink every hour. But don't forget your blood alcohol level will continue to increase for a while after your last drink too, as the alcohol is absorbed, so you might have to wait longer if you have just had another drink.

If you do choose to wait for your blood alcohol level to go down, you should check your level using a breath tester (if one is available) before you drive - just to make sure you are really under .05.

Food works in your favour

Drinking alcohol with a meal lessens the immediate effect of alcohol compared to drinking alcohol on its own. This is because having food in your stomach slows down the rate at which alcohol gets into your blood stream.

You will probably have a lower blood alcohol level if you drink while eating. The effect of food will also depend on what else you have eaten, the type of food, the type of alcoholic drink you are drinking, and the amount you eat. However eating while drinking does not prevent intoxication if you drink a large amount of alcohol, or if you drink quickly.

So although you can be pretty sure about having a lower blood alcohol level when you have your alcohol with food, the effect is so complicated that the only way to be sure that you are still under .05 is to use a breath test device before you drive. You need to remember, though, that your blood alcohol level might still increase after your last drink.

Checking your alcohol level

If you think you have had too much to drink, you probably have and shouldn't drive.

The only way to really know your blood alcohol level is to use a breath test device. A growing number of Victorian hotels, restaurants, and nightclubs now have breath testers. They may be difficult to find in some places, so it can be helpful to ask a staff member if they have one and where it is. When you are starting to think about driving, use the breath tester and find out your actual blood alcohol level.

The breath testers installed in pubs, restaurants, and nightclubs have to meet Australian Standards that ensure they are accurate. As long as you follow the instructions, you should get an accurate measurement of your alcohol level.

If you are over .05, don't drive - catch a cab or take public transport.

Don't forget if you are close to .05, you might not be safe to drive as your blood alcohol level might still be going up from alcohol you drank earlier. It's worth waiting for a bit (and not having any more alcohol) and retesting yourself to make sure your alcohol level isn't increasing.

Personal breath testing devices

If you are interested in being able to test your BAC on a regular basis, several companies in Australia sell hand held testing devices that are very accurate. The main problem is that they are also reasonably expensive and need to be recalibrated regularly to remain accurate.

You can expect to pay upwards of $800 for a robust, accurate hand held device. It is recommended to only purchase a device that meets current Australian Standards.

Some things to be aware of:

  • Don't store these devices in the boot or the glove box. Most devices are designed to be used under normal conditions. Extremes of weather such as very hot days or being in the snow may mean the devices do not work or possibly provide less accurate readings.
  • Each device operates differently in the case of a malfunction. Some devices turn themselves off or give an error message when they have malfunctioned. Ask suppliers of these devices how theirs work.
  • To remain accurate, devices must be calibrated according to the manufacturer’s instructions.