Seatbelt

Seatbelt use increased nationwide from 11% in 1982 to 85% in 2010 (NHTSA 2009, Lund 1986), largely due to the enactment and enforcement of state occupant restraint usage laws. During this time, vehicle manufacturers developed seatbelt designs to improve comfort, ease-of-use, and protection for adult occupants. Some of the improvements for adult seat belts conflicted with easy securement of child restraints. For example, a seatbelt anchorage located forward of the vehicle seat bight (the intersection of the seatback and bottom cushion) can provide a more advantageous angle for belt restraint of an adult, but makes it challenging to tighten a seatbelt adequately during child restraint installation. As a result, some issues regarding child restraint compatibility with vehicle belts and seats were addressed in SAE Recommended Practice J1819 (1994) and by the addition of a seatbelt lockability requirement to FMVSS 208 in 1996.

A common child restraint misuse with seatbelt installations is the failure of the installer to lock the seatbelt. Many seatbelts are equipped with an emergency locking retractor, which locks to prevent forward movement of the occupant during a crash event but allows movement of the occupant during normal driving. Use of a seatbelt equipped with an emergency locking retractor alone allows the child restraint to shift during normal driving. To eliminate this problem, some retractors are switchable, and can be converted to an automatic locking retractor, which allows the belt to be locked tightly through a child restraint belt path. They are usually switched by pulling the webbing completely out of the retractor and then feeding it back in to tighten.

Other belt systems use a locking latchplate that allows tight installation of the child restraint in most cases, although it can sometimes be incompatible and loosen during use. Some child restraints are equipped with belt lockoffs that can lock the seatbelt by clamping down on the webbing without use of vehicle hardware. Locking clips are still provided on child restraints without belt lockoff hardware and can be used to prevent transfer of webbing from the shoulder portion to the lap portion of the belt. However, locking clips are often ignored or placed incorrectly (instead of properly positioned within one inch of the latchplate), which may cause them to deform, fly off, and/or introduce belt slack during a crash.

Many parents or caregivers do not know that the top tether should also be used when installing a forward-facing child restraint using a seatbelt, not just when using the LATCH lower anchors.  NHTSA acknowledges the importance of tethers in their 2012 amendment to FMVSS 213: 

"Tethers provide much more secure attachment of child restraints compared to being attached by the seat belt only or lower LATCH anchors only.  In particular, they provide more rigid attachments at the top part of the child restraint, so that the CRS (CR) can 'ride down' the crash while the vehicle is crushing.  This considerably reduces the excursion of the child's head relative to the vehicle interior, so the head is are less likely to hit other parts of the vehicle interior - the most likely cause of serious injury to a properly restrained child."

Most vehicle manufacturers now state that tethers may be used in combination with a seatbelt until the child restraint manufacturer's recommended tether use limit (usually the maximum forward-facing usage weight) (2017 LATCH Manual). 

A new challenge in obtaining tight installation of rear-facing restraints using seatbelts has become more prevalent with the presence of lap-and-shoulder belts in all rear seating positions. A tight lap-and-shoulder belt can cause a rear-facing child restraint to tilt sideways as the taut shoulder belt portion pulls up on the belt path. If needed, a child restraint lockoff or a locking clip can be used to allow tightening without tipping.