With all the time, effort and frazzled nerves that are part of teaching teens to drive, parents can be forgiven for thinking that there will be one earthly reward, one thing that will make it all worthwhile. That one thing often is having the teen help shuttle younger siblings to ice-skating practice at 6 a.m. or that soccer game late on a Sunday afternoon.

Sorry, you'll have to forego that benefit.

A national study of children involved in car crashes found that having a teen-ager between the ages of 15 and 17 help with the taxiing chores is asking for tragedy.

When teens were driving, their young passengers were three times more likely to be seriously injured in a crash than when young children were driven by an adult, the study found.

"It is a dangerous practice," said Dr. Flaura Winston, the principal investigator for the Partners for Child Passenger Safety, the group that conducted the study. "Parents need to understand the excess risk."

The study looked at 12,613 crashes involving 19,111 children. Teen-agers drove 4 percent of the children in these crashes. However, when a child was injured, teens were much more likely to be driving. In these crashes, 12 percent of the children who were injured had been driven by a teen.

It is not that these teens were just driving their peers. The fact that 40 percent of these child passengers were younger than 13, suggests that teens regularly drive younger children.

A major problem is that the teens lack experience to understand how vehicles react and to recognize a dangerous situation when it arises. They may also engage in "risk-taking" behavior that increases driving speeds and results in more serious crashes.

As if that is not distressing enough, it gets worse. Teens are three times less likely to require children wear seat belts or to use the appropriate child restraints, such as car seats or booster seats, than are adults.

They were also more likely to allow youngsters to sit in the front seat, breaking one of the most basic credos of safety experts who have long said that children are safest in the back seat.

Teens are just not ready to be trusted with such "precious cargo," said Winston, a pediatrician and biomechanical engineer from the Children's Hospital of Philadelphia. The hospital conducted the study with financial help and information from the State Farm Insurance Companies.

The injury rate may also be higher because many teens are driving vehicles that are old or small and, in general, less crashworthy. These vehicles may lack airbags or seat belts with pretensioners that automatically take up the slack in the first milliseconds of a crash. Older vehicles don't have electronic stability control systems, which several important studies have shown reduce single-vehicle crashes by keeping a vehicle from skidding out of control, for example. This suggests that parents should pay more attention to choosing a safe vehicle for a teen.

Winston said what the study highlights is the need for states to adopt more restrictive regulations when it comes to allowing teen-agers to carry passengers.

Some states restrict the number of passengers a teen may transport when he or she is in the process of getting a license. But those states also exempt family members from this restriction, meaning siblings are still at risk.

Winston said the study also resurrects an issue she has been thinking about, which is educating passengers on their responsibilities for being safe. Perhaps there needs to be more of an effort to tell young passengers what is safest for them, which might also make them safer when they become drivers, she said.

The findings on teens and their passengers are part of a larger project begun in 1998 to look at what happens to children in car crashes. An earlier finding was the importance of putting children in booster seats. Young children using ill-fitting belts designed for adults were four times more likely to have serious head injuries than children using booster seats.

Booster seats are designed for children who are too big for child restraints, but not big enough to properly fit into an adult seat belt. The booster seat lifts the child, allowing the adult seat belt to fit properly across the chest and abdomen. Usually children are not ready for adult seat belts until they are at least 4 feet 9 inches tall.