At University of Michigan Transportation Research Institute, we researched the practiceof switchingseat belts to the locking mode when used with boosters as part of the development of an updated version of the UMTRI document on best practices in child passenger safety (coming soon.) When speaking with several vehicle manufacturers, most were against the practiceof lockingseatbelt with boosters becauseseatbelts are designed to allow the torso to rotate past vertical in a frontal crash, which acts to prevent submarining below the lap part of the belt when there's no crotch strap. When we have tested boosters with the 6YO and 10YO dummies, they definitely generate sufficient belt loads to activate the locking mechanism in a severe crash. I suspect that some of the reports of nonlockingbelts may be from a combination of low occupant mass andlow crash severity. The suggestion to lock the seatbelt with boosters to dealwithwiggly out-of-position kids arose years ago when there weren't higher weight harness products available. Thesewouldbe the best choice if a child can't sit properly in a booster seat. Since one of the key curriculum directives is to follow the manufacturers' directions, if the manufacturers don't tell you to lock the seatbelt with a booster, CPST's shouldn't be suggesting it as an option for parents.
Kathy Klinich University of Michigan Transportation Research Institute