Rear-facing child restraint systems

Both the American Academy of Pediatrics and the National Highway Traffic Safety Administration now recommend that children remain rear facing until they outgrow their restraint. This means that most children can remain rear-facing through age 2 years, based on average child sizes and the capacity of most rear-facing convertible restraint products on the market. Analysis of US crash data (Benedetti 2019) show that children under age two in rear-facing restraint systems have 55% lower risk of injury compared to those in suboptimal restraint systems (FFCRS or boosters or seatbelts). These recent US data support Swedish data showing benefit for children rear-facing through age 4, with rear-facing restraints reducing AIS2+ injury by 90% compared with unrestrained children (Jacobsson et al. 2007, Isaksson-Hellman et al. 1997). Because earlier rear-facing child restraints did not accommodate larger children, older education materials may contain outdated information stating that children can begin using forward-facing restraints at age 1 or 10 kg (20 lbs), which is no longer considered a best practice. Restraining an infant or toddler forward-facing too early increases the risk of injury to the spinal cord as the child’s disproportionately large/heavy head is stopped from forward motion by a tension load applied in the cervical spine.

Two types of restraints, infant restraints and convertibles installed rear-facing, are commonly used to orient the child to face the rear of the vehicle. Rear-facing restraints use an internal harness to secure the child into the shell. In a frontal impact, the restraint forces occur where the back of the child meets the restraint so that the restraining load is distributed across the entire back and head of the infant.

Infant restraints can only be used rear-facing and most have a separate base which remains in the vehicle to facilitate repeated installation, but most can also be used without the base and secured with the seatbelt. The infant restraint base can be installed with either lower anchorages or the seatbelt. These products usually have a carrying handle. Traditionally, these products have accommodated children up to 9 or 10 kg (20 or 22 lb), but there are now many models that can accommodate children up to 13 kg (30 lb) and higher.

Infant restraint installed with base

Infant restraint installed with seatbelt

A rear-facing convertible is shown below. These can be used rear-facing up to 13–20 kg (30–45 lb), then converted for forward-facing use. Convertibles tend to be larger than infant seats. While most children will outgrow their rear-facing restraint because they reach the allowable maximum weight limit for their use, some children will outgrow their rear-facing product because the top of their head is within 2.5 cm (1 inch) of the top edge of the child restraint back support. It is common practice to use an infant restraint for a newborn until it is outgrown by weight or seated height. For a product with a 9 or 10 kg (20 or 22 lb) weight limit, this means that most children would outgrow the device within the first year and should then be moved into a convertible child restraint used rear-facing.

Convertible restraint installed rear-facing

The infant’s head is well supported in rear-facing mode, and the movement of the head and neck happen in unison with the torso during a crash to eliminate severe tension and flexion forces on the neck that can occur with forward-facing occupants. The video below shows the difference in kinematics between the same child restraint used rear-facing and forward-facing in a simulated frontal impact. Peak axial neck forces are four times higher in the forward-restraint compared to the rear-facing restraint.

Using a rear-facing infant restraint facing forward can result in dangerous loading and possible ejection because the belt path has not been designed for loading in this mode. Similar consequences could occur if a convertible restraint is installed rear-facing using the belt path for forward-facing. A recent study of the dynamic consequences of misuse (Manary et al. 2019 showed that for rear-facing convertibles, the only misuse that had a significant negative effect on injury measures was routing the seatbelt or LATCH belt through the forward-facing path when installing the convertible rear-facing, which allowed excessive forward rotation during impact. Acceptable performance of rear-facing convertibles under most misuse conditions (which is not always the case for forward-facing restraint systems) may be another reason why rear-facing restraints are so effective.

Regulatory tests differ globally with regard to the extended rear-facing position. In Europe, the latest regulations (ECE R129) require either a top tether or a support leg to conform to the requirements, and do not permit forward-facing use before 15 months of age. Swedish rear-facing child restraint designs differ from US products, in that they often use a support leg and strap attachment to the front seat or are placed in the front seat against the instrument panel with the airbag deactivated to limit forward rotation of taller or heavier children. Use of a support leg has become more widespread in Europe. Some US products are now equipped with a support leg as well. However, because FMVSS 213 does not have procedures for testing with a support leg, the child restraints must pass regulatory requirements without them.

Illustration of a rear-facing CRS with a support leg

The angle of installation is one of the most critical factors for correct restraint of children riding rear-facing. If the restraint is too upright, newborn infants may not be able to breathe because their heads drop forward during travel. If the restraint is too reclined/flat, the child will not be effectively restrained by the back of the child restraint. Ensuring that the child’s head is in contact with the child restraint back support is also best for crash protection.

Focusing on crash protection, if the back support angle is more reclined than 45°, the reaction force to restrain the child in a frontal crash starts to be exceeded by the force projecting the baby upwards along the seatback and toward the front of the vehicle. As the child grows, gains weight, and can hold its head erect, a more upright restraint angle would provide better crash protection.

For the youngest infants, providing the best crash protection must be balanced with providing an angle that prevents the head from flopping over and potentially pinching off the airway. A back support angle of 45° from vertical is considered the maximum angle that can achieve these two aims. To account for the differences in vehicle seat angle, child restraint manufacturers often provide a means to indicate and adjust the installation angle. Many products now include multiple settings: a more reclined one for younger children, and a more upright angle for older children. However, indicators provided on some rear-facing child restraints may be primarily based on the angle where the restraint performs best in regulatory testing. If the installation angle required by child restraint instructions places a newborn too upright, either a different rear-facing child restraint or a car bed should be used. Some indicators may not function correctly if the vehicle is parked on a slope.

If a rear-facing restraint is installed in a rear seat with its back initially against the seat ahead, this will help limit rotation during a crash and provide improved protection, partly because the child restraint will not suddenly strike the seatback as it would if there were an initial gap (Tylko 2011, Sherwood et al. 2005). However, some child restraint manufacturers prohibit contact with the front seat because of concerns about adverse interaction between the child restraint and front seatback in a rear impact. In some vehicles with advanced airbags, vehicle manufacturers also prohibit contact between a rear-facing child restraint and the right-front passenger seat because it could interfere with occupant sensing systems.

illustration of rear-facing installation angle