In response to the challenges posed with seatbelt installation of child restraints, the National Highway Traffic Safety Administration (NHTSA) introduced a new child restraint securement system in 1999. The Child Restraint Anchorage System, commonly called Lower Anchors and Tethers for Children, is known as LATCH in the United States and is defined in FMVSS 225 and additions to FMVSS 213. The LATCH concept originated from an effort in the International Standards Organization (ISO ),
Lower anchors in vehicle seat
which proposed and adopted a universal child restraint anchorage system called ISOFIX (ISO 1999a). Implementation of LATCH in the United States began in 1999 and was required in vehicles and child restraints in 2002.

The ISOFIX concept calls for two lower attachment points and a means to “limit pitch rotation of the child restraint”. In the United States, LATCH has two distinct components: lower connectors on child restraints that attach to lower anchorage points located at the vehicle seat bight (right), and a top tether strap on forward-facing restraints that attaches to anchor points located on the rear shelf, seat back, floor, cargo area, or ceiling of the vehicle ( below).

Tether door with marking
Tether anchor behind door

Most US child restraints are equipped with a LATCH strap consisting of a length of webbing with adjustment hardware and connectors on each end. The two most common types of connectors are hook-on and push-on.  The LATCH strap is usually routed through the appropriate belt path on the child restraint that would also usually be used to route the seatbelt (figure 4 top) or attached to each side of the child restraint (figure 4 center). The LATCH strap is designed to replace the vehicle seat belt as the primary securement system. Rigid lower LATCH connectors have been manufactured on a few models of US child restraints (figure 4 bottom), but are widely used in Europe where they are required for ISOFIX. A study has shown that rigid lower LATCH connectors can be beneficial in certain type of crashes because they limit the movement of the child restraint side to side (Hauschild 2018). Attaching the top tether achieves a more secure installation and reduces occupant excursions when installing a forward-facing restraint with either the LATCH strap or vehicle seat belt. See the crash test videos here that illustrate the effectiveness of using a top tether to reduce forward head movement, even when the seatbelt is installed loosely. While using the tether improves occupant protection, child restraints in the United States must also pass less-stringent head excursion requirements without the tether to ensure reasonable protection if the caregiver fails to use it.
Hook-on lower LATCH connector

Hook-on (left) and push-on (right) LATCH strap connectors.

LATCH belt through belt path
Rigid lower LATCH connectors

Implementations of LATCH on US child restraints: LATCH strap routed through belt path (top), attached to bar on each side (center), rigid LATCH (bottom).

While many vehicles do allow easier child restraint installation with LATCH compared to seatbelts, in other vehicles the interface with the LATCH hardware makes child restraint installation difficult, and outright incompatibilities between child restraints and particular vehicles have been documented (IIHS 2003, SafeRideNews 2010). New types of misuse have been identified when using LATCH. Top tethers are only used about half the time, even though all vehicles and restraints have had ready-to-use tether hardware since 2001 (Decina and Lococo 2007, Jermakian and Wells 2010). Errors in attaching tethers include connecting them to the wrong hardware, misrouting them with respect to the head restraint, connecting them upside-down, and not tightening them sufficiently. Errors in attaching lower connectors include connecting them to the wrong hardware, connecting them upside-down, and failing to tighten the webbing after connecting. In addition, installers often install the child restraint using both the seat belt and lower LATCH strap, which is only currently allowed by one vehicle manufacturer.  Because most US products use the same belt paths to route either the LATCH strap or the seatbelt, the LATCH strap can also be misrouted through the belt paths on the child restraint.

In some vehicles, LATCH has fulfilled the intended goal of making child restraint installation easier, thus reducing misuse and improving effectiveness of the child restraint. However, because of problems in some vehicles, it may still be easier to achieve a better installation using the seatbelt. Best practice dictates that the easiest method providing a tight installation should be used to install a child restraint, keeping in mind that the tether should always be used for all forward-facing installations.

FMVSS 225 specifications include lower and tether anchorage strength requirements evaluated with a quasi-static pull test. When LATCH was first implemented, most harnessed child restraints could only accommodate children up to 18 kg (40 lb). Since then, a number of products have been introduced that allow children up to 23, 29, or even 39 kg (50 lb, 65 lb, 85 lb) to use a harnessed restraint system. Since there is no straightforward way to identify the dynamic strength limits of vehicle anchorages from the quasi-static test data, some vehicle manufacturers have expressed concern that their LATCH hardware should not be used with harnessed child seats for larger children. NHTSA clarified LATCH strength issues in a regulation stating that they consider the strength of lower and tether anchorages (based on static testing) sufficient to secure a child and child restraint with a combined weight of 65 lb (NHTSA 2012). Although many vehicle manufacturers have provided recommended weight limits (that are lower than this value) to a supplementary manual on LATCH used by child passenger safety technicians (SafeRideNews 2011), only one manufacturer currently includes any information about weight limits in their vehicle manuals (Ford 2011). While weight limits on lower anchorages may be appropriate because there is another means (seatbelt) to serve as the main method of attaching the child restraint, setting weight limits on tether use seems misguided because it is a supplement to the main attachment system, and the demonstrated benefits of top tether use in reducing head excursion in a wide range of crashes are much higher than the possible risk of injury caused by hypothetical tether anchorage failure in an extremely severe crash.