In its new report on how to reduce drunk-driving deaths, the National Transportation Safety Board states its goal in the title: "Reaching Zero." The agency thinks it is irreproachable to try to ensure that no one ever dies in an alcohol-related accident. In fact, it's a utopian goal requiring excessive compulsion in the pursuit of unattainable perfection.

The NTSB is dissatisfied by the rate of progress against drunk-driving deaths. It makes much of the fact that the percentage of traffic deaths involving intoxicated motorists has been stuck at between 30 and 32 since 1995. But that figure masks improvement.

Between 1982 and 1994, the total number of people killed in crashes involving a driver who was impaired dropped by 37 percent. In the time since, the figure has declined by 26 percent. Things are getting better—just not as quickly as before.

The earlier progress came after states lowered the blood-alcohol concentration (BAC) for driving under the influence from .10 to .08, raised the drinking age and adopted a "zero tolerance" rule for drivers under age 21. With the most potent changes already in place, public safety improvements have gotten a bit harder to come by.

One study found that lowering the BAC standard has saved some 360 lives a year. So the agency concludes that we should lower it all the way to .05, which it figures would save from 500 to 800 lives per year.

From the standpoint of individual behavior, that would be a significant change. A 180-lb. man could be legally impaired if he had three drinks in an hour (versus four drinks today) while a 140-lb. woman could earn a set of handcuffs with just two drinks in an hour (compared to three under the current rule).

NTSB says .05 makes sense because driving performance is affected even before a motorist reaches .08. That's undoubtedly true. But all sorts of factors affect driving performance, from medications and lost sleep to missed meals and electronic distractions. Not every possible impairment warrants criminal punishment.

As it happens, drivers whose BAC is higher than .01 but lower than .08 account for only 5 percent of all highway deaths. The agency estimates that a motorist with .05 BAC is 38 percent more likely to crash than one who is stone-cold sober. That sounds like a lot until you realize that someone with a .08 BAC is nearly three times more likely to be involved in a wreck than an abstainer. At .10, the risk is nearly five times greater.

Not that a 38 percent increase is trivial. But is it worth the cost to prevent it? Nothing in law enforcement comes free, and resources expended on enforcing a lower limit are resources diverted from other worthy purposes.

Under a tighter BAC, the same number of cops will be chasing a lot more offenders. An officer who is busy arresting someone with a .05 level, who poses a small danger, will not be able to arrest someone with a .10 or .15 level, who poses a huge danger.

Forgotten in these number-crunching exercises is the effect on the trivial matter of human spontaneity and enjoyment. NTSB may place a zero value on the pleasures of social drinking, but social drinkers don't. In any cost-benefit analysis, the loss of a largely harmless pleasure deserves inclusion.

Not only that, but the cost of a stricter policy is greater than it may appear. When you lower the limit, you deter not only forbidden levels of consumption but permissible levels. Why? Because most people don't care to be anywhere close to the line.

They don't travel with Breathalyzers and want to be absolutely sure of not being arrested for DUI—which carries not only legal penalties but social disgrace. If this were a constitutional right, we'd be talking about a "chilling effect."

It may come as a surprise to hear that the organization that deserves much of the credit for raising public awareness of the problem, Mothers Against Drunk Driving, has declined to endorse this proposal. It prefers to focus on greater efforts to enforce existing laws, while requiring ignition interlocks for every DUI offender.

NTSB acknowledges this last policy would save some 1,100 lives per year—far more than a lower BAC would save. It also has the virtue of disabling the few guilty without inconveniencing the many innocent.

In a free society, trying to reach zero carries too high a cost. Better to settle for making progress.