“If they’re old enough to serve in the military, they should be old enough to drive.”

“I was drinking when I was his age. They’re just having fun.”

“If the kids are drinking at my house, at least I know where my own kids are.”

When it comes to drinking and older teenagers, relaxed parental attitudes can contribute to the problem. In a national study of 11th and 12th graders commissioned by Liberty Mutual and Students Against Destructive Decisions, nearly half of teenagers said they were allowed by their parents to go to parties where alcohol was being served at least rarely, and more than a third said they were allowed to drink when their parents were with them. Slightly less than a third reported being allowed to drink unsupervised. The numbers are up slightly from those in a similar study done in 2011.

While it’s possible to be of two minds about the legal drinking age, few parents are ambivalent about the question of teenagers drinking and then getting behind the wheel. More than one in 10 students reported driving under the influence of alcohol or drugs after New Year’s Eve in the same study (all the more disturbing since by definition, these aren’t drivers who have had many opportunities to do so).

With young college students home on break, this holiday period of socializing and parties is a good time for parents to revisit the contract they may have had in place with their teenager regarding drinking, drugs and driving, suggests Dave Melton, managing director of global safety for Liberty International and an impassioned advocate for keeping young drivers safe. (Families without contracts can find them at the Centers for Disease Control’s Teen Driving Web site: Parents Are the Key.)

Recognize that some teenagers will be drinking, says Mr. Melton. Renew your promise to pick up a child anytime, anywhere, no questions asked. “It’s not a matter of getting angry,” Mr. Melton said. “It’s a matter of getting them home.”

The same study found a number of positives: a majority of teenagers said they would stop drinking and driving if asked by a passenger, or would speak up and tell an impaired friend not to drive. But parents need to encourage teenagers to act on those good intentions. We all know there can be a big distance between what we say we will do in any given situation and what we actually do. There may also be an impossible-to-measure gap between a teenage driver whose driving ability is actually impaired and one who thinks he is, or whose friends think she isn’t O.K. to drive. Those differences can be a matter of life and death this New Year’s Eve, and any other night as well.

Have you asked your teenager to make a New Year’s resolution, and a promise to you, not to drive under the influence of drugs or alcohol and to assume any driving friend who has had even a small amount to drink (or used other substances) isn’t safe to drive?

Contact KJ Dell-Antonia at:

kj.dellantonia@nytimes.com