Kuala Lampur, May 28 -- "I'm okay. I only had a little."

It is a phrase heard far too often after dinners, celebrations, office gatherings, and late-night outings. Sometimes it is said jokingly, sometimes confidently, and sometimes with genuine belief. The person saying it may even walk steadily, speak clearly, and feel fully capable of driving home.

But alcohol does not merely affect movement or speech. One of its most dangerous effects is that it quietly impairs the brain's ability to judge its own impairment.

That is what makes driving under the influence so dangerous. The problem is not simply that alcohol slows reflexes or affects coordination. The greater danger is that many people genuinely believe they are still functioning nor...