Reducing Risks to Children in Vehicles With Passenger Airbags

Children Should Be Restrained Properly

Tremendous resources have been devoted to increasing child restraint and belt use over the past 20 years.²⁴ Rates of child restraint use in the United States have risen from 20% in 1980 to 79% in 1996.^{25 26} The most recent data found that 93% of infants, 75% of toddlers, and 65% of subteens were restrained,²⁶ although this study did not measure the extent of misuse. Currently, approximately half of the deaths among infants and toddlers in the United States annually occur in crashes in which children are not restrained in child restraint devices.²⁴ Children from less educated, low-income, and minority families are less likely than other children to use child restraints and safety belts.²⁷

Misuse of child restraint devices is a widespread problem, observed in as many as 80% of the children restrained in child safety seats.^28-32 Proper restraint refers here to the use of age- and weight-adequate restraint systems, the adequate restraint of the child to the system, and the adequate restraint of the child seat or booster to the vehicle seat.

Proper restraint use reduces a child's chances of being killed or seriously injured in a motor vehicle crash, even in vehicles without passenger-side airbags. Even accounting for misuse, the magnitude of the protective effect is substantial, probably greater than the 45% reduction in fatality risk attributed to adult use of lap and shoulder belts.^{24 33-35} Proper child restraint use also dramatically reduces a child's risk of being killed or seriously injured by a passenger-side airbag (except if the child is in a rear-facing child seat). A properly restrained forward-facing child, positioned with the seat back as far as possible, has a minimal risk of death or serious injury from deployment of passenger-side airbags.¹

The most promising strategy for achieving higher levels of proper restraint use is to replace secondary with primary enforcement laws, thereby permitting police officers to apprehend and fine drivers who allows occupants to ride in their vehicle unrestrained. Unrestrained drivers are particularly likely to allow their children to ride unrestrained in the front seat.^{36 37} Thus, efforts to increase adult restraint use, which remains <65% in the United States,²⁶ may induce increased child restraint use. Controlled studies have demonstrated that highly visible educational and enforcement efforts can increase the rate of adult and child restraint use in a community.^38-40 In the future, more uniform and improved designs of child restraint systems (eg, uniform seat mountings and top tether requirements) should reduce misuse and facilitate compliance with child passenger safety laws.

Although efforts to increase the rate of proper child restraint should be a major priority, this strategy alone is unlikely to be completely successful in protecting children from passenger-side airbags. Additional increases in child restraint use will come slowly. The residual unrestrained child population in the United States will be difficult to reach without substantial and well-targeted resources. Moreover, the widespread problem of misuse is more complicated to solve through education and is difficult for police to detect. The fact that several restrained children have been killed and seriously injured by passenger-side airbags cautions us against complete dependence on child restraint use and safety belts as measures to protect children from the dangers posed by passenger-side airbags.³⁵

Secure Children in the Rear Seat

Approximately one third of US children younger than age 12 traveling in motor vehicles are seated in the front seat.^{36 41 42} The front-seating position is used more frequently by infants and toddlers than by young children and subteens. In the 1996 Controlled Intersection Study, 71% of infants and 67% of toddlers were observed riding in the front seat (NCSA, NHTSA, personal communication). However, these estimates may be upwards, and the percentage of infants and toddlers in the front seat has been declining gradually for 10 years.⁴¹

It is feasible to move large numbers of children from the front seat to the rear seat. In continental European nations, such as Germany, France, and Belgium, traffic laws prohibiting children from riding in the front were enforced from the mid-1970s to the early 1990s, well before the introduction of passenger-side airbags in Europe. In recent years, these laws have been relaxed following guidance from the European Commission.⁴² Yet, it is still customary in many European countries for children to ride in the rear of the vehicle, even if the vehicle has only two occupants (a driver and a child). A recent roadside survey of vehicles occupied by at least one child in Frankfurt, Brussels, and Paris found <15% of such vehicles with a child in the front seat.⁴³ Even in the United States, where no such laws (until very recently) have existed, some states have twice the proportion of children riding in the front seat as other states.⁴¹

It is well established that the rear seat is intrinsically safer than the front seat for an occupant of any age.^{44 45} In the absence of a passenger-side airbag, a child's average risk of fatality is 20% to 40% lower in the rear compared with the front seat.^{13 35} The risk of injury also is diminished in the rear seat.^{34 45 46} If a passenger-side airbag is present in the vehicle, the net safety advantage of the rear seat for children is enlarged, for both restrained and unrestrained children.¹³

The NHTSA recommended recently that states amend child passenger safety laws to require that children sit in the rear when a seat is available.⁴⁷ Rhode Island became the first state to enact such a law, albeit only for children younger than age six. Several states are now considering similar legislation.⁴⁸ Recent surveys have found that a large majority of parents (including those with young children) have favorable attitudes toward the idea of requiring children to sit in the rear.²² Reticence about new legislation in this area appears to revolve around three concerns discussed below.

First, parents with large families and car-pooling arrangements may have no choice but to allow one or more children to ride in the front seat. However, shortage of rear seating capacity is rarely a valid explanation for children riding in the front. Of the 17 000 fatal crashes involving US children seated in the front from 1985–1996, empty seats in the rear were available in >95% of the vehicles and, in 65% of the vehicles, the rear seat was completely unoccupied.⁴¹ However, any laws on this matter should apply only to vehicles that have a rear seat and only in circumstances in which the rear seat is otherwise unoccupied.

Second, some older vehicles are equipped with only a lap belt in the rear seat; thus, it may seem safer to seat the child in the front, where both a shoulder belt and a lap belt are always available. The available data on this issue are limited. It may be appropriate to restrict the applicability of child seating laws to the vast majority of vehicles on the road in the United States that are equipped with lap and shoulder belts in the outboard rear seating positions. At a minimum, such laws should apply to any vehicle that is equipped with a passenger-side airbag, because all such vehicles also are equipped with lap/shoulder belts in outboard front and rear positions.

Third, to legislate where parents must seat their children in a vehicle may be viewed as an excessive intrusion on the part of government into the realm of parental choice. However, similar objections could be made against the existing child passenger safety laws that compel parents to purchase child restraint devices and use these devices in specified ways. Child seating laws would seem to have at least as much philosophical justification—rooted in the state's interest in protecting the rights and welfare of children—as child restraint use laws.

As additional resources are devoted to enforcing adult and child passenger safety laws in the states, it may be appropriate to coordinate such efforts with amendments to such laws that compel children to ride in the rear seat. Police officers will find it easier to observe where a child is seated than to observe whether a restraint is used and used properly. Laws against improper child seating position will provide police a clear and indisputable rationale for stopping a vehicle, thereby making more feasible stringent enforcement of child and adult restraint use laws. Once a vehicle is stopped, a police officer can make a more effective observation of restraint use in each seating position.

Suppress Airbag Deployment Through Active or Passive Measures to Protect Children

If significant numbers of children continue to ride in the front seat, it may become necessary to disarm passenger-side airbags to protect these children.

Raise Deployment Thresholds for Airbags to Prevent Unnecessary Firings

When children experience fatal injuries from airbag deployment, the crashes often are relatively low-speed impacts that would otherwise not have been fatal.¹⁴ This empirical observation led to the suggestion that deployment thresholds for airbags be raised from their current level of 12 miles per hour into a fixed barrier, with a range from 7 to 15 mph (or ∼11 to 24 km/hour) to a considerably higher level (eg, near 18 mph or 30 km/hour). A case can be made that the threshold for firing on the passenger side should be set even higher than the threshold on the driver side, because there is no steering wheel on the passenger side.

Vehicle manufacturers and suppliers are currently reexamining where airbag deployment thresholds should be set. This decision is not governed directly by the NHTSA's regulations, allowing manufacturers design flexibility in this area.

A potential drawback to raising deployment thresholds, without improving the performance of crash sensors, is that airbags will take longer to deploy, particularly in so-called soft crashes (eg, a crash against a utility pole), where the crash pulses are attenuated and spread over time. A longer time interval from crash to bag inflation gives more opportunity for adults and children to slide forward into the airbag's deployment zone. As sensor performance is improved, either through placement of additional satellite sensors in the front of the vehicle or through advanced sensor technology, it should be feasible to raise deployment thresholds without delaying airbag deployment. A complementary solution may be to block airbag deployment if the time required to trigger inflation exceeds some preset time, gauged as the time during the crash that an unrestrained passenger takes to move into the airbag's deployment zone.

Depower Airbag Systems and/or Install Dual-stage Inflators Linked to Crash Severity

Manufacturers selling vehicles in Canada and the United States already have received permission from regulatory agencies to depower airbags by 20% to 35%.¹ Depowering refers to decreasing the energy of deployment and/or reducing the volume of the bags themselves. A study in the United States found little evidence that adult occupants who died in frontal crashes involving airbags died because their airbags had insufficient powering.⁵² Some experimental models suggest that depowering can reduce significantly the rate and severity of airbag-induced injuries to children and adults.¹ In Australia, where rates of seat belt use exceed 90%, some depowered airbags, which are also small in volume (eg, the Holden bag), are in use. Early real-world data from Australia suggest that depowered airbags, in conjunction with other design features, inflict fewer and less severe injuries without sacrificing protection in high-speed crashes.³

Another notable refinement is dual-stage inflation. Once a collision is sensed and is of low-velocity, a first reduced charge of propellant is deployed into the airbag. Milliseconds later, if deceleration associated with what turns to be a more severe collision is detected, a second charge of propellant is released that makes the airbag larger and/or firmer. For decades, this approach was considered promising because it would mitigate airbag-induced injuries to out-of-position children and adults in low-speed crashes.^{15 28 53-56} With this design, two deployment thresholds are required rather than one. An early version of the dual-stage inflator was used on the passenger side of 10 000 General Motors cars equipped with airbags during model years 1974–1976.⁵⁷ An elaboration of dual-stage inflation might entail allowing only first-stage deployments when a child is occupying the front passenger seat.

In recent offerings, BMW and Mercedes-Benz include two different thresholds of deployment based on whether the occupant is belted.⁵⁶ A properly belted occupant is less likely to need airbag protection in relatively low-speed impacts than is an unbelted occupant. Despite these innovations, the BMW owners manual recommends that children younger than age 12 sit in the rear.⁵⁸

There exists a formidable barrier to protecting children through use of precise adjustments in the inflation power of airbags. The child dummies currently used by suppliers, manufacturers, and government may not provide an accurate indication of the vulnerability of a child's head, brain, and neck, which may vary significantly from child to child. The minimum force on the neck necessary to fatally or severely injure a child is not very different from smaller forces on the neck that would not be associated with significant injury.^59-61This narrow window needs to be defined precisely for children of different vulnerabilities and reflected in sophisticated child dummies.

Any child whose head is extremely close to the airbag when deployment occurs (<1 inch) is likely to suffer serious or fatal injury as long as the airbag is powered sufficiently to provide protection for large adults in a moderate-speed crash. A tradeoff between adult and child passenger protection has been recognized by airbag engineers for >25 years.⁶² Dual-stage inflation, coupled with improved sensor technology and improved child dummies, should make this trade-off more favorable, but will not eliminate it.