The drunk driver is a villain. We have seen the wrecked cars and the wrecked lives. Every Christmas, we read the newspaper articles about people victimised by drunk drivers - the lost children, parents, brothers, sisters, aunts and uncles.

So why do people drive when they are drunk? The obvious answer is that they're alcoholics - except it's not true. Two-thirds of "drunk drivers" involved in fatal accidents are light to moderate drinkers, according to Stephen Rowen of the Rutland Centre, the Dublin addiction-treatment clinic, quoting US research.

Garda statistics do not include the number of people involved in accidents when they were drunk. Nor do we have any statistics on the drinking habits of those caught drunk-driving. Such information is essential if we're serious about preventing such behaviour. In its absence, we have to rely on surveys from other countries to indicate trends.

Most drunk drivers are not addicts out of control. They are people like us. So we shouldn't be asking why other people drive when they are drunk. We should be facing the harder question of why we drive when we are drunk.

Rowen thinks the image of the drunk driver as an alcoholic villain is part of our denial mechanism. We don't need to be alcoholics to get tipsy, then get in a car. "People have worked all day, then they go to a party, where they eat and drink more than they planned to. They're going home at 1 a.m. or 2 a.m., and sometimes they just fall asleep behind the wheel."

Marie Murray, head of psychology at St Vincent's Hospital in Fairview, says that most people who drive drunk don't include themselves in the drunk-driver category. "We see drunk drivers as 'other people', not us. There are people who drive with too much drink who would be appalled if you described them as drunk drivers. It's not the blind drunk who are most dangerous - people will take the keys from them.

"The dangerous driver is the person who has had too much, but it's not discernible. They have slower reaction times, are less responsive and lose their ability to interpret danger. They don't stop and think this is an appalling thing to do, to risk other people's lives and to risk your own life."

Our refusal to accept that anyone who drinks and drives is, effectively, a drunk driver feeds into all kinds of psychological defence mechanisms, such as rationalisation - "sure, I only had one or two" - and projection - "I'm no more drunk than anybody else."

Drinking gives us a feel-good illusion of competence, well-being and invincibility. This is why we are able to convince ourselves that we will get home safely - haven't we done it before, after all? We also know our chances of being caught are tiny.

Despite the hype around Christmas drink-driving campaigns, a mere 9,570 people were arrested for driving while over the limit in 1999, up 1,000 on the previous year - a statistic the Garda press office quotes with pride.

It may seem a big number, but it isn't. Divide it by 365 and we see that only 26 people a day are arrested for drink-driving in the entire country. "Twenty-six arrests per day is hardly significant, considering that there are 13,000 licensed premises, including off-licences, used by hundreds of thousands of people every week," says Rowen. He believes the Garda isn't taking the problem seriously enough to make checkpoints a real deterrent. "Garda∅ in tight communities go into pubs and empty them out at closing time. People get into their cars to drive home, and the garda∅ let them."

When we take the risk of driving home after a few drinks, some of us are running through the maths. "We try to calculate how much we can drink without being criminally irresponsible," says Murray. We know a drink - a glass of beer or wine or a shot of liquor - takes 45 minutes, on average, for the liver to metabolise. If we eat a high-protein meal while we drink, we metabolise the alcohol even faster.

There are dozens of websites that allow us to calculate the effect of alcohol on our bodies, so that if we follow the rules we will probably test under the limit on a breathalyser, should we be so unlucky as to encounter a checkpoint.

All of this rationalisation is ridiculous, however, because getting into a car after drinking - even with a meal and coffee afterwards - cannot be a rational calculation. "Don't try it," warned a Garda press officer. "If you're drinking, don't drive at all." Murray agrees. "If you're going to a social gathering where you'll be drinking, or tempted to drink, don't take the car."

The problem is that, although cars and alcohol are a dangerous mix, they are both essential to our social lives. "You shouldn't drive after even one drink. Yet anybody, even a professional person, is prepared to take chances. We've all done it," says Ann-Marie McMahon, a counselling psychologist at St John of God Hospital, in Dublin.

"Anyone who drives a car after even one drink is a drunk driver. I've been in the situation where the taxi doesn't turn up, so in desperation I take the car, because there's no other way of getting home. Or you go to a party and the husband, who promised not to drink, decides to have 'just one pint', and before you know it he's legless. You have only had a couple of drinks, so you take the keys and drive home. For people not to be put in this situation, we'd need minicabs running all over Dublin over Christmas."

This is where we get into broader social issues, not only about taxi availability, but also about our conviction that a party is not a party without booze. Alcohol consumption has risen by 41 per cent during the past 10 years, and alcohol advertising subliminally encourages people - the young, especially - to drink in order to become uninhibited. Advertising uses slogans such as "How wicked are you?" and "As clear as your conscience".

"We need a major, paradigmatic shift in attitudes," says Murray. "We live in a culture that is particularly steeped in alcohol as a way of expressing our emotions. If we choose not to drink, we are actually being dissonant with our own culture. We equate getting drunk with having a good time. You see this even more intensely with young people who are intentionally going out to get plastered. Cars, alcohol and sex are at the centre of an increasingly important subculture."

The young feel even more invincible than the rest of us. Speed, alcohol and youth account for most fatalities, but Murray fears that advertisements such as the anti-speeding campaign in which a woman warns, "Slow down boys", put it up to the "boys" to drive faster.

As well as using drink for entertainment, we use drink to self-medicate, to ease tension and to deal with social phobia and anxiety - conditions that affect as many as one in 10 people. Some people drink at home, then get into the car because they cannot otherwise face social gatherings. This doesn't mean they are alcoholics, simply that they are afraid to seek, or cannot find, help to deal with their anxieties, says Murray.

At the core of it all is the fact that most of us who drink cannot see ourselves being affected by the drink we take, because we're not alcoholics. We like the way we feel when we drink. It gives us feelings of well-being and invincibility. We think we're in control, yet we cannot imagine going to parties over Christmas without drinking. "At what stage does a culture grow up and not need to souse itself?" asks Murray.

We're a long way from that stage. Meanwhile, the act of drinking clouds judgment, so every time we drive after even one drink we are taking a risk. Without a sufficient Garda presence on the roads to act as a real deterrent, we are prepared to take these risks. We are dicing with death every time we put alcohol into our bodies, then put the keys in the ignition.

Tomorrow, in the continuing series on road carnage, Kathy Sheridan examines the human cost of crashes, when she visits the National Rehabilitation Hospital, in D·n Laoghaire