Seatbelts

Overview

The term seatbelt refers to either a lap-and-shoulder combination or a lap belt alone. In the U.S., lap-and-shoulder belts have been required in all rear seating positions since 2007. However, there are still some vehicles on the road that may have only lap belts in center rear seating positions. Vehicle seatbelts are designed primarily with adults in mind, and geometric factors may make good fit difficult for children. However, if a more appropriate restraint system is unavailable, seatbelts provide some protection even for small children, and effectiveness rates for seatbelts are calculated for occupants age 5 and up to be near 50% in terms of reducing fatal injuries (Wiacek et al. 2011). For children aged 11-13, risk of injury for unbelted children is 5 times the risk of a child wearing a lap-and-shoulder belt (Benedetti et al. 2019.) Seatbelts are part of the continuum of restraint systems with varying levels of effectiveness for children. In general, more restraint is better than less, and good fit is important for effective restraint performance. Unfortunately, poor fit of seatbelts often leads to misuse, with shoulder belts placed behind the back or under the arm (Louman-Gardiner 2008, Gotschall et al. 1998, Meissner et al. 1994) which degrades their performance and increases the likelihood of submarining and belt-induced injury.

Child Size and Belt Fit

Belt fit depends on the size and posture of the occupant, the size and shape of the vehicle seat, and the geometry and features of the belt system. A child who has good belt fit in one vehicle may not in another. A child’s pelvis is generally shorter, less calcified and less prominent than that of an adult. A child can locate the top of their own pelvis by finding the bony points at the top front of the pelvis. The lap belt should lie completely below these points to ensure that the lap belt can contact and restrain the pelvis during a crash (below). Good fit of a lap belt is as low as possible on the pelvis, touching or flat across the tops of the thighs. However, it the belt is positioned too far forward on the thighs, the child can move forward before the belt begins restraining through the pelvis. The shoulder belt should lie flat on the shoulder about halfway between the neck and the arm and cross the chest at the middle of the sternum.

A common recommendation is that children should not use a seatbelt without a booster until they reach a standing height of 148 cm (58 in) and a clothed weight of 37 kg (81 lb) based on an early study of booster belt fit (Klinich et al. 1994). This size corresponds to a 90th percentile 9-year-old, a 50th percentile 11-year-old, and a 5th percentile 13-year-old; based on current growth charts, the majority of children 10 and under would benefit from using a booster seat (Klinich et al. 2016). While a simple height, age or weight recommendation is convenient for educational or legislative purposes, several studies indicate that most children above this stature still experience better belt fit with a booster. Analysis of field data indicate that children aged 5 to 10 using boosters have a 30% lower risk of injury than those restrained only by seatbelts (Benedetti et al. 2019).

To achieve the best seat belt fit, the child should be sitting fully upright with their pelvis as far back into the seat as possible, and preferably with their feet touching the floor. This will help place the lap belt in front of the pelvic bone below the anterior-superior iliac spines and will minimize the possibility of the belt sliding up and intruding into the soft upper abdomen. Several studies have shown that children tend to move forward on the vehicle seat to allow their knees to bend comfortably over the front edge of the seat, causing the child to slouch. This rotates the pelvis rearward, making it more difficult for the lap belt to engage the pelvis, and can lead to the lap belt being positioned over the abdomen. If a child cannot achieve an upright, seated posture, or if the shoulder belt crosses the throat, the child needs to use a booster.

Shoulder belts that touch the side of the neck are not likely to cause injury unless they are very loose (Kortchinsky et al. 2008, Corben and Herbert 1981, Appleton 1983). However, discomfort from a shoulder belt against the neck tends to cause the child to put the shoulder belt under his or her outboard arm or behind the back. The shoulder belt should not be routed behind the child’s back because it offers no torso restraint and tends to pull the lap portion of the belt upwards on the inboard side, both of which increase potential for injurious belt loading (Brown and Bilston 2007). Also, with most belt designs, routing behind the back eliminates the loading of the shoulder belt early in the crash sequence, which, on a properly worn belt, functions to snug the lap belt and, in some retractor designs, lock the belt. Finally, the shoulder belt should never be routed under the arm, because the resulting belt forces on the side of the thorax are known to result in serious internal injuries in a crash (Gotschall et al. 1998, States et al. 1987).

It is possible for shoulder belt loading to cause thorax injury in severe crashes as it loads the child (Jermakian et al. 2018). To reduce likelihood of injury from belt loading, advanced seat belt features which have been implemented for front-row occupants are gradually being introduced in the rear seating positions. One of these features is a pretensioner, which removes slack from the seatbelt when a crash event is detected. Another feature is a belt load limiter, which allows the shoulder belt to spool out further once a particular load threshold is reached. An airbag mounted in the shoulder belt to provide better load distribution over the thorax has been introduced on rear-seat belts in some vehicles. Designs for airbag restraints to protect rear seat occupants in frontal crashes have been proposed. Vehicle manufacturers have also added seatbelt features to improve fit for various sizes of occupants. Many vehicles have an adjustable shoulder belt anchorage that can be raised or lowered to better route the belt over the occupant’s shoulder. Some have positioning guides or loops that can also help provide better fit for smaller occupants. However, these may not help fit problems with lap belts or vehicle seat cushions that are too long for a child to sit upright comfortably.

Shoulder Belt Positioners

Various unregulated devices have been marketed to move a shoulder belt away from a smaller occupant’s neck. Most of these products pull the shoulder belt into position by anchoring a device to the lap belt, thereby pulling that portion of the belt upward and gaining shoulder belt fit at the expense of proper lap belt fit (Brown et al. 2010, Sullivan and Chambers 1994). Unlike a belt-positioning booster, shoulder belt positioners typically pull the lap belt up onto the abdomen as they pull the shoulder belt down and away from the neck. In addition, they do nothing to improve the posture and slouching of a child too small to fit in the vehicle seat. Because pediatric dummies cannot currently measure loading to the abdomen, evaluation of the potential negative effects of shoulder belt positioners cannot be quantified. These products may be packaged with misleading claims that they “meet all relevant standards” when none apply. Shoulder belt positioners should not be used in place of belt-positioning boosters, which are proven in the field to reduce injury, particularly to the abdomen.

Lap versus Lap-and-Shoulder Belts

Fortunately, the relatively recent requirement to provide lap-and-shoulder belts in all vehicle rear seating positions has reduced the need to use only lap belts to restrain occupants. The principles of restraint theory lead to the conclusion that lap-and-shoulder belts would be better for children, even if fit is not optimal, than a lap belt alone. Analysis of fatality and injury data confirms that lap-and-shoulder belts are 13-15% more effective than lap belts alone. While lap-only belts reduce the risk of ejection and injury, they increase the risk of abdominal injuries; lap-and-shoulder belts reduce the risk of both head and abdominal injuries (Benedetti et al. 2019, Elliott et al. 2006b, Mulpuri et al. 2007, Morgan 1999).

Lap-and-Shoulder Belts and Airbags

Even with advanced airbags, which can sense and adjust deployment to the size and type of front passenger, parents are warned not to have children under age 13 ride in the front seat of a vehicle. Older vehicles such as small pickup trucks without advanced airbags may have on/off switches for frontal airbags. In situations where a child must ride in the front seat with an active airbag, because no switch is available and the back seat is filled, a child in a seatbelt may be at greater risk of injury from the frontal airbag than a younger sibling restrained in a forward-facing child restraint. This is because the child using the lap-and-shoulder belt is able to lean forward in their shoulder belt or even put the belt behind the back. This behavior may place the child’s head in the path of the deploying airbag or allow their upper body to be thrown forward during precrash braking.

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