D.C. police will begin enforcing a new law next month that requires children to ride in a car safety seat until the age of 8, making the District the only jurisdiction in the country to impose such a requirement.

Child safety advocates have lobbied for stronger child-restraint laws in recent years, citing studies showing that adult seat belts do not provide enough protection for children ages 4 to 8. Maryland and Virginia recently amended their laws, making car seats compulsory for children younger than 6 instead of under 4.

The District, however, is the first jurisdiction to require a child safety seat until age 8, according to the National Highway Traffic Safety Administration's most recent data. The new measure, which passed the D.C. Council in June and received congressional approval last week, replaces a D.C. law that required safety seats for children younger than 3.

"This is a fantastic step," said council member Adrian M. Fenty (D-Ward 4), who introduced the bill. "Some may criticize this law as increased government intrusion, but we have to interfere. It is just too dangerous to leave children with the possibility of being in danger."

D.C. police began setting up checkpoints last week to inform the public about the new law and to issue warnings to violators. On Dec. 22, police will start writing tickets. First-time offenders will face a fine of $75 and two points on their operator's license, but the fine will be reduced to $25 if they attend a child restraint safety class.

The law will apply to any motorist driving in the District, including residents of Maryland and Virginia, said Bill Rice, the spokesman for the District's Department of Transportation.

Rice and others said that police officers for the most part will accept a driver's word on the age of children riding in the car, the same policy they used in enforcing the old child-restraint law. They said the goal of the legislation is to educate parents on their children's safety rather than to write tickets.

"The purpose is not to fine or penalize anybody, but to make vehicles safer for children," Fenty said.

Auto safety advocates call youngsters ages 4 to 8 the "forgotten children" because they are often thought to be too big for a child booster seat even though many are still too small to be protected by an adult seat belt.

According to the NHTSA, 13 percent of the children ages 4 to 14 who died in motor vehicle crashes in 1999 were restrained by adult safety belts.

When small children are buckled with an adult lap belt, it lays across their abdomen and stomach rather than their hips. Those are "soft areas, and in case of collision, the pressure could cause internal injury," said Heather Sitterding, who coordinates the child transportation safety program in the Virginia Department of Health.

Sitterding also said that adult shoulder belts often come across small children's faces. "So they tuck it behind their back, and in that case, they have no upper body restraint, and that could cause head injuries," she said.

NHTSA's guidelines recommend that children ride in a booster seat with lap and shoulder belt until age 8, unless they are 4 foot 9 or taller. Safety advocates in Virginia and Maryland said that they agree with that recommendation but that they could not persuade their state's lawmakers to approve such a stringent requirement.

"We had to compromise. It was either this or nothing at all," said Barbara Backet, state coordinator for the Maryland Safe Kid Coalition, a national nonprofit organization.

Maryland's new law, which will take effect Oct. 1, 2003, requires safety seats for children younger than 6 and also for older children if they weigh less than 40 pounds. Virginia's new law took effect July 1 and, like the District's, is based solely on the child's age.

At the federal level, Congress has sent President Bush a bill to require automakers to install children's lap and shoulder belts in the rear center seats of all new cars by 2005. The bill is known as "Anton's law," named after Anton Skeen, a 4-year-old who was killed in a car crash in Washington state in 1996.

Safety experts say the rear middle seat is the safest place in the car for children because it protects them from exploding front-seat air bags and keeps them as far away as possible from the impact of a collision.

According to NHTSA, many parents fail to install child car seats properly. Under the new D.C. law, revenue from fines in excess of $55 will be used to fund seat-fitting stations in each of the city's eight wards. Currently, there are only four fitting stations in the District.

The District Department of Transportation's Web site lists 14 locations in the city, including Children's Hospital, where parents can buy toddler car seats for $25 and infant and booster car seats for $15, which is cheaper than the price at most stores. In addition, police are distributing a limited number of free seats to low-income families.