People know they shouldn't drink and drive, but that's when they're sober. After a few drinks, driving is one of the many bad ideas that can suddenly make sense. What, me, impaired? I feel fine.

A new study finds that people's attitudes about drinking and driving change after they've been drinking. And it suggests that the best time to remind people not to drink and drive is before they start drinking.

If you want to find out why people decide to get behind the wheel after they've been drinking, you need to study them after they've been drinking.

That's why the best way to prevent drunk driving remains for people to decide that they're not going to drive before they start drinking. It's much better than making this decision while they're fuzzy. They can plan on taking a taxi, public transportation, or using a designated driver.

To study how people react under stress, you need to observe them while they're under stress. Similarly, if you want to find out why people decide to get behind the wheel after they've been drinking, you need to study them after they've been drinking. So that's what researchers did.

A group of university students participated in two separate sessions. In the first session they described how dangerous it was to drive after they had been drinking. In the second, they drank a moderate amount of alcohol (about 0.7g/kg body weight) and reported their willingness to drive at various times after drinking the alcohol.

They reported the idea of driving after drinking as less dangerous after they had drunk alcohol. And after drinking, they were more willing to drive than they indicated they would have been while they were sober. The difference between students' ideas about drunk driving and their sense of their own driving was largest when their blood alcohol was decreasing.

The findings suggest that it's not enough for people to be aware of the risks of drinking and driving or even to understand exactly how much alcohol it takes for them to be an unsafe driver. They also need to understand it while they're drunk. Or at least know they can't trust their belief in their own driving ability while they're tipsy.

Reminding drinkers not to drive shortly before they're ready to go home might be effective. But how well this would work isn't clear because it's not known whether the message will get through to the alcohol-fuddled brain or not. That's something a future study might reveal.

Drinkers can take matters into their own hands by telling themselves that they're absolutely not going to drive before they set out to drink, and then follow through by taking a taxi or public transportation to their favorite watering hole or assigning a designated driver before they go. This is the safest way to ensure that they won't drive after they've been drinking, no matter how good an idea it may seem to be later on in the evening.

The study appears in Alcoholism: Clinical & Experimental Research.