Recommended use and effectiveness

When a child no longer fits in a harnessed restraint, the next step is a belt-positioning booster seat used with a vehicle lap-and-shoulder belt. As with the transition from a rearward-to forward-facing child restraint, this step to a booster actually decreases the level of occupant protection offered and should be delayed as long as possible. Boosters do not restrain children. Instead, they reposition the child and redirect vehicle belts (designed to fit adults) to be routed appropriately relative to the child’s body. Both the NHTSA and the AAP recommend that children use booster seats until they fit in seat belts alone, which means most children should be using boosters through age 8–12 years (AAP 2011). Booster seat use among 4-to-8 year olds has risen to 63% in 2007 from 15% in 2000, largely as a result of state laws requiring their use, public education programs, and more available booster products (NHTSA 2009).

Children aged 4–8 using boosters are 45% less likely to sustain injury in a crash compared to children using seatbelts alone (Arbogast et al. 2009, Durbin et al. 2003). Boosters are particularly effective at reducing abdominal injury: children using belts alone are 8 times more likely to sustain abdomen injury than children using a belt-positioning booster with the vehicle seatbelt (Jermakian et al. 2007). The video below illustrates consequences from simulated frontal crashes for a 6YO with and without a booster. With a booster, the lap belt loads and restrains the strong bones of the pelvis. Without a booster, the dummy slides under the lap belt, so the belt loads the abdomen, vulnerable internal organs, and spine instead of the pelvis. This event is often called “submarining” under the lap belt.

Link to video with and without booster (coming soon)

How boosters improve belt fit

Over the past decade, evaluations with child volunteers have examined how different booster seat designs improve belt fit using realistic vehicle and seat belt geometries (Reed et al. 2008, 2009, Bilston and Sagar 2007). This research has led to a better understanding of how booster seats improve The first thing that any booster seat does is raise the child up relative to the vehicle belt as shown below. Even if the booster does not have a back, the elevation helps position the shoulder belt away from the neck so it is more comfortable and restrains the child through the shoulder structure in a crash.

When considering the lap belt, shifting the child upwards relative to where the lap belt is anchored increases the lap belt angle. The steeper lap belt angle is better because it makes it harder for the child to slide under the lap belt in a crash.

The second way boosters work is by improving occupant posture. Several studies have documented that the rear seats of most vehicles are too deep for children to sit upright with their knees bending over the edge of the seat and with their back fully supported for comfort (Huang and Reed 2006, Klinich et al. 1994,  Bilston and Sagar 2007). Consequently, children scoot forward so their legs can bend over the front of the seat in a comfortable position, as shown on the left. Using a booster seat provides them with a cushion length that is more compatible with their upper leg length (center) and provides an upright posture similar to that of an older child (right).

The third way boosters work is by routing the seatbelt using lap-and-shoulder belt guides. The lap belt should be positioned so it is completely below the top of the pelvis, which reduces the likelihood that it will slide up over the abdomen in a crash. Well-designed lap belt guides help position the belt so it touches the top of the child’s thighs, and resists upward movement of the belt in a crash. Well-designed shoulder belt guides position the shoulder belt midway between the neck and arm, not at the edge of the shoulder or rubbing the neck. Neck injury from the shoulder belt contacting the neck has not been identified as a problem in the field. The biggest danger from the shoulder belt touching the neck is that it could cause the child to put the shoulder belt under the arm or behind the back. Either misuse virtually eliminates upper-body restraint that the properly placed shoulder belt would provide. In one study of booster misuse, 20% of children improperly placed the shoulder belt behind the back or under the arm (O’Neil et al. 2009), which can lead to poor outcomes (O'Neil et al. 2012). Poorly designed shoulder belt guides can pull the shoulder belt too far off the child’s shoulder, or allow slack to develop after a child leans forward because it interferes with easy retraction of the shoulder belt.

Changes in booster use and design

There are currently four styles of belt-positioning boosters: backless boosters (left), removable-back boosters (center), highback boosters (right), and built-in boosters. Backless boosters can be used when the vehicle seat and head restraint support the child’s head to the tops of the ears. Some backless boosters have an optional shoulder belt guide on a strap to adjust the shoulder belt position if necessary. With removable-back boosters, the lower portion can be used alone or with a booster seatback. Highback boosters are usually constructed as combination seats that can be converted from harnessed restraints. A few vehicle manufacturers provide integrated booster seats that fold out or pop up from the vehicle seat (Jakobssen et al. 2007).

Results from field data show that there was no difference in injury risk between boosters with and without backs (Arbogast et al. 2009). While boosters with backs have features that could improve protection in side impacts and may keep children in a better position laterally relative to the vehicle belt system particularly when sleeping, backless boosters allow children to sit further rearward, which effectively reduces head excursion. From a practical standpoint, backless boosters and built-in boosters allow children to enjoy the comfort features of a vehicle seatback, and since they are not as visible from outside the vehicle, they may be preferred by older children reluctant to use a booster. In addition, one study of children in boosters showed that children seated in products with large side wings for improved side-impact protection leaned forward 55% of the time compared to 25% of the time for children seated in boosters with less prominent side wings (Andersson et al. 2010). 

While many boosters with backs have shoulder-belt positioning devices that improve static belt fit, research has indicated that the devices are not that effective at keeping the shoulder belt position in place during dynamic loading (Tylko and Dalmotas 2005,Klinich et al. 2008). Based on these results, it is best to choose a booster and vehicle seating position that achieves good shoulder belt fit with minimal redirection of the shoulder belt by the booster. As shown below, it would be better to have a straighter line path between the D-ring and shoulder (right) than one substantially rerouted by the booster (left).

Booster seats must meet dynamic FMVSS 213 requirements using a test bench equipped with only one defined lap-and-shoulder belt geometry.  However, lap-and-shoulder belt geometry in the rear seats of vehicles can vary widely. Some boosters may not be able to route belts with a particular geometry so the belt will fit well on a particular size of child. In practice, the best approach is to evaluate the belt fit with the specific child, vehicle seating position and booster seat. Several studies have documented that the effectiveness of the booster seat routing features varies with vehicle belt geometry (McDougall 2011, Brown et al. 2009). The Insurance Institute for Highway Safety has developed a rating system for assessing the belt fit across a range of vehicle belt geometries (Reed et al. 2009). However, given the effectiveness of booster seats demonstrated in field data and the many factors that allow boosters to improve belt fit, any booster is likely to provide better seat belt fit for a child than the no booster condition.

Some children making a transition from a harnessed restraint to a belt-positioning booster often have trouble staying correctly positioned for the entire trip, as the shoulder belt’s emergency locking retractor comfort features allow considerable movement unless activated and locked during a crash. If a child will not stay in position, some have suggested locking the shoulder belt with its switchable retractor (if available). However, this does not allow enough forward motion of the torso, which prevents submarining under the lap belt in the absence of a crotch strap. A child who is not developmentally ready to sit still in a booster would be better protected in a high-weight harness child restraint.

Securing a booster in a vehicle

When first introduced, belt-positioning booster seats were not secured to the vehicle, as their purpose is to position a child relative to the vehicle seat belt, but not to actually provide restraint. However, the lack of attachments sometimes allowed the boosters and child to shift during driving and caused instability during loading/unloading. Since many caregivers do not fasten the seatbelt around the booster when unoccupied as directed, a loose booster could be a projectile in a crash. There are some booster products designed to allow the booster to be secured to the lower anchorages and/or tether anchorages with the LATCH hardware. This is most common among boosters that convert from a harnessed restraint to a belt-positioning booster and thus have LATCH attachments. Some boosters also have rigid or flexible LATCH attachments solely to hold them in place (SafeRide News 2011). This practice has not been universally adopted, because there are lingering concerns among some manufacturers that if the booster and seatbelt but not the child are attached to the vehicle, the child could slide forward on the booster and have a greater risk of injury than if the booster moved with the child. Testing results with boosters attached to vehicles have been mixed, with some tests showing improved kinematics using a LATCH-secured booster, and others showing less desirable
kinematics (Tylko et al. 2005, Transport Canada 2011).